What sin accrues to us, the three brothers, by virtue of being legal brothers of the adopted son, even though we are not?

Adoption, which is the attribution of a person to other than his biological father, is forbidden in Islamic law. The registration of lineage in official documents does not change the religious ruling. Thus, the adopted man is neither a son nor a brother to the family's daughters, and they must observe hijab in his presence, just as they would with any other non-mahram.