Short answer. No, not intestate. Article 992 of the Civil Code erects a barrier: an illegitimate child has no right to inherit by intestacy from the legitimate children and relatives of the father or mother, and those legitimate relatives cannot inherit from the illegitimate child in the same way. This applies to half-siblings across the legitimate-illegitimate divide.
What the law says
An illegitimate child has no right to inherit ab intestato from the legitimate children and relatives of his father or mother; nor shall such children or relatives inherit in the same manner from the illegitimate child.
Civil Code, Article 992 — The Barrier Between Legitimate and Illegitimate Relatives. Read the full provision →
The legal barrier Article 992 creates
Article 992 of the Civil Code is often called the 'iron curtain' rule of Philippine succession law. It cuts off intestate inheritance between two categories: illegitimate children on one side, and legitimate children and relatives of the illegitimate child's father or mother on the other. The rule is bilateral — the illegitimate child cannot inherit from the legitimate relatives, and those legitimate relatives cannot inherit from the illegitimate child. Neither side may inherit from the other by intestate succession.
How this applies to your half-sibling
A half-sibling who is illegitimate on your father's side shares your father but was born outside marriage. You, as a legitimate child or relative of your father, are exactly the kind of person Article 992 protects — and bars. Your illegitimate half-sibling cannot inherit from you by intestacy, and you cannot inherit from them by intestacy. The blood connection through your father is not enough; the law treats the legitimate and illegitimate family lines as separate for inheritance purposes.
The will as an alternative route
The barrier Article 992 creates is an intestate succession rule — it governs what happens when there is no will. It does not prevent inheritance by will. If your illegitimate half-sibling wants to leave you something, or you want to leave them something, a valid will can accomplish that — subject to the rules on legitimes (the mandatory shares of compulsory heirs). Article 992 cannot be circumvented without a will, but a properly executed will can make the gift.
Practical implications
If you or your illegitimate half-sibling dies without a will, Article 992 will apply. The illegitimate child's own descendants would be their intestate heirs — not their legitimate half-siblings. If you want to ensure something passes between you and your half-sibling, both of you need to make wills naming the other. Consider also whether any assets might transfer outside succession — through joint ownership, beneficiary designations on insurance, or co-ownership — as these are not governed by the same rules. Consulting a lawyer is the safest way to plan for these situations.