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

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, sallallaahu ‘alayhi wa sallam, 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.