Marriage with woman with child for earlier marriage

Aslaam-o-aleikum! I am planning to InshaAllah marry a woman, we both are sunni muslims. But she has a 8 year old boy from her earlier marriage. We live in a joint family, my brother has a baby girl and my mother also lives in the same house. The question is when I get married, her son will live with us. When he becomes an adult (18yrs) will he be a na-mehram for my brother daughter or my mother? Is it legal in islam to live in a joint family with a "son" I do not have any blood relation with? Jazakallah kheir.