Though the marriage was void, he had taken the children in and gave them his name. Therefore, they are now his legitimate children and the boy is Mahram to you.

 I will be getting married to a revert man who has two children from his previous marriage while he was still a Hindu with a Muslim woman. Their marriage was a civil marriage as he did not convert. They get divorced and the two children are with the mother.  My concern is are these two children (a boy and a girl) considered his children or illegitimate? Is he responsible for the two children? And if I were to marry him, this revert man, is his son a mahram to me? Can we stay under one roof? What about his daughter? Is he a mahram to her? If I were to have children with him in future what are the relations between my children and his children? Please help me to clear my doubts. I do not want to live in sin.