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.
It is permissible for you to live with her son, though you should consider that in shariah, the custody for an underage child does not fall upon the mother if she has remarried and instead goes to any other female relative of the child who is not married to a stranger to the child. See this answer for details and references.
After puberty the boy and your brother's daughters will not be mahrams, though he wouldn't be a mahram for her even if he was your biological son (cousin marriages are permissible in Islam). Additionally, regardless of the boy, your brother will also not be a mahram for your wife (Sahih Muslim on Brother-in-law).
It is permissible for non-mahram family members to live in a single house, provided that they properly cover themselves in each other's presence(Quran 24:31) and are not left alone with each other in seclusion (Bukhari and Muslim). Also see answer on islamqa.info).
A joint family isn't an Islamically recommended practice. Islam requires you to provide a separate residence for your wife as part of her nafqah (maintenance), and at the very least she should have a separate room to herself. (see Hidayah, islamqa).