A Salam 'Aleykoum. Before getting married with my mother my father had a son with a non Muslim woman. I decide to keep in touch with him again, because I learnt that in Islam family links are very important. But after talking to him on the internet I made some research and found out that he was not my brother from an Islamic point of view. But is he still a mahram? I told myself that yes, because genetically we have the same father so we can't get married, but I prefer to ask someone of knowledge to be sure. So can I shake his hand and take off my hijab (and do all the thinks women are alowed to do when with mahram) in front of him? Thank you for your answer and may Allah soubhannah wa ta'ala bless you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If this boy is the son of your father from Zina (fornication or adultery) – we seek refuge in Allaah from it – then he is not a Mahram to you. Therefore, it is not permissible for you to be in seclusion with him, or to touch him, or to show your adornment in his presence; rather, he is like all other non-Mahram men to you [and you should consider him so].

Allaah Knows best.