Q. A man had an affair with a woman from whom a son was born (out of Nikah). Can that son be a Mahram to the man’s daughter born in Nikah?
A. Shariah does not recognize a child born out of the wedlock of Nikah through Zina (adultery/fornication) to be in the same category as a child born in wedlock (Nikah).
If a man committed adultery and a son was born outside the wedlock of Nikah, the illegitimate son cannot serve as a Mahram to the man’s legitimate daughter for purposes of travel. He is also prohibited from marrying her. (Hindiyyah 6/455)
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa
This answer was collected from TheMufti.com, which is a fatwa portal managed by Mufti Ismaeel Bassa from South Africa.