What is the ruling of Islamic law on a relative adopting an illegitimate child and naming him after her husband, then trying to bequeath her apartment to him despite her sisters' refusal, especially with the potential existence of legitimate parents for the child, and how can she be convinced to retract this?

Adoption in the legal sense is forbidden by Islamic law and contradicts the Quranic verse: "And He has not made your adopted sons your [true] sons," and His saying: "Call them by [the names of] their fathers." However, guardianship (kafala) and upbringing, while the fostered individual retains his lineage, are permissible. What the questioner's relative did, if it was adoption, is a grave sin, and she must repent and rectify its consequences. The claim of having no heir is incorrect, as sisters do inherit, and the adopted young man is not an heir to her. She may, however, bequeath to him within the limit of one-third [of her estate]. She must treat the young man as a non-mahram (definition) unless he was suckled by her with the suckling that establishes mahramiyya (prohibition of marriage).