Sheikh `Alî A. al-Jumu`ah, professor at al-Imâm University, Qasîm Branch
Adoption in Islam does not change a person's true lineage and it does not establish ties of kinship. Allah says: “Nor has He made your adopted sons your sons. Such is (only) your (manner of) speech by your mouths. But Allah tells (you) the truth and He shows the (right) way. Call them by the names of their fathers; that is juster in the sight of Allah. But if you know not their father’s names (then they are) your brothers in faith or your wards.”
Therefore, the man who adopted her is not one of her close relations who are allowed to see her without her wearing hijâb. Among those who are allowed to see the woman without hijâb are the people mentioned in the following verse where Allah says: “And not display their beauty except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers or their brothers’ sons, or their sisters’ sons, or their women, or the slaves whom their right hands possess, or male attendants free of sexual desires or small children who have no carnal knowledge of women” [Sûrah al-Nûr]
She may also be seen by the close relations who are stipulated in the in Sunnah, these being her uncles and those who are related to her through fosterage due to suckling. The Prophet (peace be upon him) said: “The forbidden degrees of marriage due to suckling are the same as those due to kinship.”
In the case of this man, if his wife, his daughter, sister, son’s daughter, daughter’s daughter or his niece nursed this woman in her infancy, then he is mahram to her and she will not have to observe hijâb in his presence. This man will be a father in the first case, a grandfather in the second, a maternal uncle in the third, a grandfather in the fourth and fifth, and a maternal uncle in the sixth due to suckling. Otherwise, he has no kinship relationship with her whatsoever.
Source: Islam Today