All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, you should know that adoption is forbidden and whoever adopts a child does not become his real parent. Therefore, this man is not your brother, so it is permissible for you to marry his daughter.
Rather, this man should be traced back to his real father. If he does not know his real father, he could be given a general name like the son of 'Abdullaah. Also, he is a non-Mahram to your mother, so she has to wear Hijab in front of him and she should not allow him to be in seclusion with her.
Allah Knows best.