The stepson of a brother is not a Mahram to his sisters

Question 10: My brother got married to a woman who had a son from a previous marriage. My brother brought up this stepson, who was an orphan, since the latter was only two years old. He is twenty five years old now. Do I have to wear Hijaab (veil) in front of him bearing in mind that my brother took charge of him and raised him in his orphanage?

Answer: The concerned boy is not your brother’s son unless your brother’s wife breastfed him after being married to your brother or that your father’s wife did so. In such a case, the boy referred to in the question is considered your foster brother… read more here.