What is the religious ruling on the inheritance of an adopted child from an illegitimate relationship, who is legally registered as the daughter of the deceased, and is her inclusion in the inheritance declaration without invalidating her lineage considered participation in a forbidden act?

The matter of lineage is serious, and therefore, Islamic law has established rules to preserve it and prohibited delving into the honor of Muslims. A distinction must be made between adoption and the acknowledgment of an unknown lineage. Adoption is invalid in Islamic law, whereas the acknowledgment of lineage is valid under certain conditions.

If the father acknowledges a young girl of unknown lineage as his own, her lineage is established by this acknowledgment, and no one has the right to invalidate it, not even the father himself if he later denies it—his denial will not be accepted. Accordingly, this girl is your sister; she inherits from you, is unlawful for you to marry, and her name must be recorded in the inheritance declaration. However, whoever knows or strongly suspects the invalidity of her lineage must treat her as a stranger.

As for the case where the father adopted the girl but did not acknowledge her as his own, or if she has a confirmed lineage from someone else, then the father has committed a major forbidden act. Invalidating her right to lineage and inheritance requires referring the matter to the Sharia court for consideration.