Fathwa, - A woman's father's son from Zina is..

Question
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.
Answer
All perfect praise be to Allaah, The Lord of the Worlds. I testify
that there is none worthy of worship except Allaah, and that
Muhammadis 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.