Fathwa, - Step-brother is not a Mahram to his..

Question
Salam alaikum I have a grown up girl of 12, and i am considering
marriage to a man who has joint custody of his 13 yr old son. This
means his son will stay 4 days with him. What is the ruling behind the
situation esp if the son will be living int eh same house as my
daughter who would be a step sisters. jazakalla khair i look forward
to recieving your answer
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 Muhammad,
is His Slave and Messenger.
If you marry that man and he has a son from another wife, then his son
is a non-Mahram )marriageable man( to your daughter, and thus it is
not permissible for them )i.e. his son and your daughter( to be in
seclusion with each other or to mix together in a suspicious manner.
Rather, they have to be considered strangers to each other.
Allaah Knows best.