1) What “Relative by Affinity” Means (and Why It Matters)

A. Consanguinity vs. Affinity

Consanguinity: relationship by blood (parents, siblings, cousins).
Affinity: relationship by marriage—your spouse’s relatives become your relatives by affinity, and your relatives’ spouses also become related to you by affinity.

Examples (common in “in-law” situations):

Father-in-law / mother-in-law: 1st degree (direct line, ascending) by affinity
Stepchild / stepparent: usually treated as 1st degree by affinity (created by marriage)
Brother-in-law / sister-in-law: 2nd degree (collateral) by affinity
- includes both: (1) your spouse’s sibling and (2) your sibling’s spouse

B. Degrees of affinity (practical guide)

In Philippine civil law, the degree by affinity generally mirrors the degree of blood relationship between your spouse and the spouse’s relative.

Quick examples:

Spouse ↔ spouse’s parent = 1st degree affinity
Spouse ↔ spouse’s sibling = 2nd degree affinity
Spouse ↔ spouse’s niece/nephew = 3rd degree affinity
Spouse ↔ spouse’s first cousin = 4th degree affinity

C. Why “affinity” is legally relevant

A marital affair with an in-law is not automatically a “special crime” just because of the family connection—but affinity can matter in at least five major ways:

Criminal liability for extramarital sex (adultery/concubinage), and sometimes VAWC
Civil/family remedies (legal separation, damages, property recovery)
Marriage prohibitions (attempts to marry certain in-laws can be void and can trigger bigamy issues if a prior marriage still exists)
Succession and property transfers (void gifts/donations; possible disqualification under wills in specific circumstances)
Sexual offenses where affinity can be a qualifying circumstance (especially if minors, coercion, or incapacity are involved)

2) What Counts as a “Marital Affair” Legally

A “marital affair” is a social term. Philippine law treats different behaviors differently:

A. “Emotional affair” (no sex proven)

Usually not adultery/concubinage (those require sexual intercourse or specific cohabitation/scandal elements).
May still support civil/family actions (e.g., legal separation for “sexual infidelity” can be harder without proof of sexual acts, but patterns of conduct may be relevant; VAWC psychological abuse may be possible in some factual settings).

B. Sexual affair (sex, cohabitation, scandal, or a second “relationship home”)

Can trigger adultery or concubinage depending on who is married and what acts can be proven.
Can support legal separation and other consequences.

3) Criminal Actions Potentially Available

3.1 Adultery (Revised Penal Code, Art. 333)

Who commits it

A married woman who has sexual intercourse with a man not her husband.
The male partner is liable if he knew she was married.

Key points

Sexual intercourse must be proven. Suspicion, chats, and closeness alone typically won’t satisfy the elements.
Each act can be treated as a separate offense in principle.
The fact that the partner is a relative by affinity (e.g., brother-in-law) does not change the statutory elements, but it may affect evidence, credibility, and damages.

Who can file / how it is prosecuted

Private crime: generally must be initiated by a complaint of the offended spouse (the husband).
The complaint must usually include both offenders (the spouse and the paramour), not just one.
Consent/condonation/pardon can bar or defeat prosecution depending on timing and circumstances (including situations where the offended spouse knowingly resumes marital relations after learning of the affair).

3.2 Concubinage (Revised Penal Code, Art. 334)

Who commits it

A married man under any of these circumstances with a woman not his wife:

Keeps a mistress in the conjugal dwelling, or
Has sexual intercourse under scandalous circumstances, or
Cohabits with her in another place.

Key points

Concubinage is not proven by a single secret encounter unless it fits one of the statutory modes (dwelling/scandal/cohabitation).
The mistress/partner faces a different penalty (historically “destierro” in the statute), while the husband faces a correctional penalty.

Who can file / prosecution

Also treated as a private crime: generally requires complaint by the offended spouse (the wife).
Common barriers: consent, condonation, or pardon; and rules requiring inclusion of both parties.

3.3 Bigamy (Revised Penal Code, Art. 349) and “Trying to Marry the In-Law”

If a married person goes through a marriage ceremony with someone else while the first marriage is still valid and subsisting, bigamy may apply—regardless of whether the new partner is an in-law.

Why affinity can intensify the legal mess

Certain marriages involving relatives by affinity are void as against public policy under the Family Code (see Section 5 below).
Even if a subsequent marriage is void, bigamy risks can still arise when a person contracts a second marriage while the first has not been legally ended/invalidated under Philippine law.

3.4 Violence Against Women and Their Children (RA 9262) – When an Affair Becomes a Criminal Case

Scope RA 9262 penalizes physical, sexual, psychological, and economic abuse committed against women (and their children) by a person who is or was in a specified relationship with the victim (e.g., husband, ex-husband, boyfriend, former partner).

How an affair may connect An affair by itself is not automatically the crime. But an affair may become part of:

Psychological violence (mental or emotional anguish), especially where there is humiliation, harassment, threats, repeated deception, public ridicule, or cruelty tied to the affair; and/or
Economic abuse, e.g., withholding support, dissipating resources, giving away community/conjugal assets to the affair partner, or abandonment patterns that cause suffering.

Important

RA 9262 is designed to protect women and children. It is not a symmetric remedy for husbands against wives.

3.5 When “Affinity” Can Elevate Sexual Offenses (Not “Affair,” but Often Overlooked)

If what is labeled an “affair” includes:

Coercion, lack of consent, intimidation, or exploitation; or
A minor or someone unable to validly consent,

then Philippine sexual offense laws may apply. Affinity can become a qualifying circumstance in some sexual crimes (e.g., qualified rape under the Revised Penal Code as amended by RA 8353, where the offender is a relative by consanguinity or affinity within a specified degree, among other qualifiers). The exact applicability depends heavily on facts (age, consent, relationship, circumstances).

3.6 Criminal pitfalls in “gathering evidence”

In trying to prove an affair, complainants sometimes expose themselves to criminal liability or inadmissible evidence issues, including:

Anti-Wire Tapping Act (RA 4200): recording private communications without authorization can be unlawful.
Anti-Photo and Video Voyeurism Act (RA 9995): recording or sharing sexual content without consent is criminal.
Cybercrime Prevention Act (RA 10175) and Data Privacy Act (RA 10173): hacking accounts, unlawful access, or misuse of personal data can create serious exposure.
Defamation (libel/slander): public accusations without proof can backfire.

4) Civil and Family-Law Actions (Non-Criminal Remedies)

4.1 Legal Separation (Family Code, Art. 55 and related provisions)

Ground

Sexual infidelity is an explicit ground for legal separation (Art. 55).

What legal separation does (and does not do)

It allows spouses to live separately and can separate property relations.
It does not dissolve the marriage bond (spouses generally cannot remarry).
It affects property, custody, and inheritance rights in specific ways.

Timing

An action for legal separation must generally be filed within five (5) years from the occurrence of the cause (Family Code rule on prescription for legal separation actions).

Defenses / bars Legal separation may be denied where there is:

Condonation (forgiveness),
Consent,
Connivance (active participation in wrongdoing),
Collusion (fabricating grounds),
Reconciliation (which can extinguish the action),
Prescription (late filing).

Property consequence A decree of legal separation can result in forfeiture of the guilty spouse’s share in the net profits of the property regime, generally in favor of common children (or otherwise as the law provides).

4.2 Declaration of Nullity / Annulment – How an affair fits (and usually doesn’t)

Philippine law does not list “infidelity” as a direct ground to nullify/annul a marriage. The common pathways are:

Void marriages (e.g., lack of essential requisites, incestuous marriages, those void by public policy, etc.)
Voidable marriages (e.g., lack of parental consent at the time, fraud, force/intimidation, incapacity, serious STD, etc.)
Psychological incapacity (Family Code, Art. 36), which is not “mental illness” in the ordinary sense and requires a structured legal/clinical showing.

Where the affair might matter

As evidence of deeper traits (e.g., persistent, grave relational incapacity) in an Art. 36 case, depending on the pattern and totality of evidence.
As evidence of misconduct relevant to property/custody disputes.

4.3 Damages (money claims) against the spouse and/or the third party

Even without (or alongside) criminal prosecution, civil claims may be pursued under:

Civil Code Art. 19 (abuse of rights),
Art. 20 (acts contrary to law causing damage),
Art. 21 (acts contrary to morals, good customs, public policy causing injury),
Potentially Art. 26 (privacy, peace of mind), depending on conduct.

Against the cheating spouse

Claims commonly center on humiliation, mental anguish, and bad-faith acts that go beyond “mere marital disappointment,” especially where there’s cruelty, public scandal, harassment, or financial betrayal.

Against the third party (including an in-law)

A third party is not automatically liable just because an affair happened. Liability tends to turn on bad faith, malice, and injury—for example, deliberately intruding into the marital relationship in a manner that is clearly contrary to morals/good customs and causes demonstrable harm.
The fact the third party is a close relative by affinity can strengthen arguments about betrayal, bad faith, and aggravated injury, but it is still fact-driven.

Damages in criminal cases Because criminal liability generally carries civil liability, damages may also be claimed within criminal prosecutions, subject to proof and procedural rules.

4.4 Recovery of property given to the affair partner (especially important in “in-law” affairs)

Affairs often involve transfers of money, vehicles, real property, tuition payments, travel, etc.

Key legal hooks:

Civil Code Art. 739: Donations are void when made between persons guilty of adultery or concubinage at the time of the donation (and in other listed situations).
Civil Code (testamentary succession provisions commonly paired with Art. 739): certain dispositions by will in favor of persons guilty of adultery/concubinage with the testator can be attacked, depending on the specific statutory rule and proof.
Family Code rules on community/conjugal property: disposition of community property often requires proper authority/consent; unauthorized transfers can be void/voidable depending on the property regime and facts.

Practical consequences:

Money and property funneled to a paramour may be recoverable or the transfer may be void, especially when it can be framed as a prohibited donation, fraudulent conveyance, or unauthorized disposition of community/conjugal assets.

4.5 Child-related consequences: custody, support, and paternity disputes

Affairs with an in-law can produce intense family conflict affecting children. Legally:

Support obligations remain.
Custody decisions are anchored on the best interests of the child, not as “punishment” for infidelity (though conduct that affects parenting capacity, safety, stability, or exposes the child to harm can matter).
If the wife has a sexual affair during marriage, paternity/legitimacy issues may arise, but Philippine law applies strong presumptions and strict timelines and grounds for impugning legitimacy.

4.6 Succession and disinheritance angles

Infidelity can have estate implications:

Legal separation grounds can intersect with disinheritance rules (the Civil Code lists causes by which a spouse may be disinherited, and causes for legal separation can be relevant).
Illicit-relationship-related donations and certain testamentary dispositions can be challenged under specific provisions.

5) Special Issue: Can the Spouse Later Marry the “Relative by Affinity”?

Even apart from criminal adultery/concubinage, an “affair with an in-law” often leads to the question: “Can they marry later?”

A. Philippine rule: divorce is generally not available (with limited exceptions)

As a general rule, a valid marriage is not dissolved by divorce under ordinary civil law. Status changes typically come from:

Declaration of nullity (void marriage),
Annulment (voidable marriage),
Legal separation (does not allow remarriage),
Recognition of a foreign divorce in limited situations (notably where a foreign spouse obtains a divorce abroad and the Filipino spouse is capacitated to remarry under the applicable rule),
Muslim Personal Laws for those covered by the Code of Muslim Personal Laws (which has its own rules on divorce and related matters).

B. Void marriages by reason of public policy (Family Code)

The Family Code treats certain marriages as void for public policy reasons, including specific relationships by affinity (commonly understood to include):

Between a parent-in-law and a child-in-law, and
Between a stepparent and a stepchild, among others.

So, even if a person becomes legally “free to marry” later (through recognized means), marriage to certain in-laws may still be void due to these prohibitions.

C. Bigamy risk if someone “marries the in-law” without being legally free

If the original marriage remains valid and no recognized legal process has severed or invalidated it, going through another marriage ceremony can create bigamy exposure, even if the new “marriage” is void for other reasons.

6) Strategy and Proof: What Usually Makes or Breaks These Cases

A. Different standards of proof

Criminal cases (adultery/concubinage/bigamy): proof beyond reasonable doubt.
Civil/family cases (legal separation, damages, property recovery): typically preponderance of evidence (or other civil standards depending on the proceeding).

B. Evidence commonly used (legally obtained)

Admissions (messages, written statements) obtained without unlawful access
Witness testimony (observations of cohabitation/scandal)
Hotel/lease/utility records in proper cases
Financial trail (transfers, purchases, receipts)
Photographs/videos lawfully obtained (with strong caution on privacy laws)

C. Common legal obstacles

In concubinage: proving cohabitation, conjugal dwelling, or scandalous circumstances
In adultery: proving actual sexual intercourse, not just intimacy
In legal separation: prescription (5 years) and condonation/consent/connivance
In third-party damages: proving bad faith and a clear, compensable injury

7) Putting It Together: What Actions Are Typically “On the Table” (Checklist)

When a married person has an affair with a relative by affinity (e.g., brother-in-law/sister-in-law), the possible actions—depending on the facts—commonly include:

Criminal

Adultery (if the married spouse is the wife and sex is provable)
Concubinage (if the married spouse is the husband and statutory modes are provable)
Bigamy (if a second marriage is contracted while the first remains valid)
RA 9262 (where the affair is tied to psychological/economic abuse against a woman/child)
Sexual offense cases if consent/age/incapacity issues are present (where affinity may aggravate/qualify)

Civil / Family

Legal separation (sexual infidelity ground; subject to time limit and defenses)
Damages (Civil Code Arts. 19/20/21/26 theories; fact-sensitive, especially vs. third party)
Property recovery / nullification of transfers (void donations; unauthorized disposal of community/conjugal assets)
Child-related relief on support/custody/visitation, guided by best interests and statutory duties
Estate consequences: disinheritance angles; invalidation of certain donations and possibly certain testamentary dispositions in illicit-relationship contexts

Key Takeaways

An affair with an in-law is not a “special” standalone offense, but it triggers existing criminal and civil frameworks.
Adultery and concubinage have strict elements; “messages and suspicion” alone often fall short.
Legal separation squarely recognizes sexual infidelity as a ground, but it has a 5-year filing window and multiple defenses.
Affinity becomes especially important when the parties attempt to marry later (void marriages by public policy; bigamy risk), when transfers of property are made (void donations), and in certain sexual offenses where affinity can be a qualifying circumstance.
Evidence must be gathered carefully—privacy, wiretapping, cybercrime, and voyeurism laws can create serious liability if violated.