In the Philippines, succession is governed primarily by the Civil Code, which dictates how an estate is distributed upon a person's death. Central to this system is the concept of Legitime—that portion of the testator’s property which is reserved by law for compulsory heirs. The testator cannot deprive these heirs of their legitime, except in cases of valid disinheritance.

Crucially, the law distinguishes between "compulsory heirs" (who are entitled to a legitime) and "voluntary or legal heirs" (who may inherit through a will or by operation of law in the absence of one).

The Status of Siblings vs. Illegitimate Children

Before examining the partition, a vital legal distinction must be made regarding the status of these relatives:

Illegitimate Children: These are compulsory heirs. Whether the deceased left a will or not, illegitimate children are entitled to a specific portion of the estate.
Siblings: Brothers and sisters are not compulsory heirs. They have no right to a legitime. They only inherit in Intestate Succession (when there is no will) and only in the absence of compulsory heirs in the direct descending and ascending lines (children and parents).

Table of Legitime for Illegitimate Children

The legitime of illegitimate children varies depending on who else is surviving the deceased. Under the law, the share of an illegitimate child is generally one-half (1/2) of the share of a legitimate child.

Surviving Heirs	Legitime of Illegitimate Children (IC)	Other Heirs' Legitime	Free Portion
IC Only	1/2 of the total estate	N/A	1/2
Legitimate Children (LC) + IC	Each IC gets 1/2 of the share of one LC	LC get 1/2 of the estate	Remainder (taken from Free Portion)
LC + Spouse + IC	Each IC gets 1/2 of the share of one LC	LC: 1/2; Spouse: Share of 1 LC	Remainder (taken from Free Portion)
Legitimate Parents + IC	1/4 of the total estate	Parents: 1/2 of the estate	1/4
Spouse + IC	1/3 of the total estate	Spouse: 1/3 of the estate	1/3

Note: The "Free Portion" is the part of the estate the deceased could have given to anyone through a will. If the total legitimes of illegitimate children exceed the free portion, their shares are reduced pro-rata.

Rules on Partition for Siblings (Intestate Succession)

Since siblings are not compulsory heirs, they only participate in the partition of the estate through Intestate Succession. This occurs if the deceased died without a will, or if the will is void.

1. When do siblings inherit?

Siblings inherit only if the deceased leaves no legitimate descendants, no legitimate ascendants, and no illegitimate children.

2. Concurrence with a Surviving Spouse

If the deceased leaves a surviving spouse and siblings (or nephews/nieces), the estate is partitioned as follows:

Surviving Spouse: 1/2 of the estate.
Siblings (or their children): 1/2 of the estate.

3. Full-blood vs. Half-blood Siblings

The Civil Code provides a specific ratio for partition among siblings of different parental connections:

Full-blood siblings receive a share double that of half-blood siblings.
Example: If the estate is ₱60,000 and there is one full-blood brother and one half-blood sister, the brother gets ₱40,000 and the sister gets ₱20,000.

The "Iron Curtain" Rule (Art. 992)

A critical restriction in Philippine law is Article 992, often called the "Iron Curtain" rule. It prohibits reciprocal intestate succession between illegitimate children and the legitimate relatives of their parents.

An illegitimate child cannot inherit ab intestato (without a will) from the legitimate children or relatives of his/her father or mother.
Conversely, the legitimate relatives (including siblings) cannot inherit from the illegitimate child.

This rule emphasizes that the legal "line" between the legitimate and illegitimate family is impenetrable in the absence of a will.

Summary of Intestate Shares

When there is no will, the law provides for the "Legal Shares," which are often larger than the "Legitime."

Surviving Relatives	Share of Illegitimate Children	Share of Siblings
IC + LC	1/2 share of one LC	Zero (Excluded by children)
IC + Spouse	1/2 of the estate	Zero (Excluded by children)
Spouse + Siblings	N/A	1/2 (Shared among siblings)
Siblings Only	N/A	100% (Divided among them)

Key Takeaway for Partition

If a person wishes to leave property to a sibling while they have children (legitimate or illegitimate), they must execute a Last Will and Testament. Without a will, the children will exclude the siblings entirely from the inheritance, regardless of the emotional closeness of the relationship.