My sister married a man who has boy (aged 17 years) from his previous wife. The man divorced my sister but the boy (who is not my sister's son) is living with my sister in our joint family home. In Islam can this boy live with my sister as her step-son? Can he be treated as her son? Or should she observe hijab around that boy?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The stepmother is a Mahram for the step-son, and she does not have to put Hijab in front of him. She can appear in front of him in the same state she appears in front her brother and father, on the condition she does not cause him temptation.

However, if this son is living in a joint family home where there are other women, other than the wives of his father, then it is not permissible for these women to appear in front of him and it is not permissible for him to be in seclusion with them or to shake hands with them. He is a Mahram for his step-mother but not for the sister of his step-mother or the like.

Allah knows best.