Question 5: From `Abd-al-`Azeez ibn `Abd-Allaah ibn Baaz, to the honorable brother the inquirer, may Allaah guide him to every goodness. Aameen.
Al-salaamu `alaykum wa rahmatullaah wa baraakatuh (May Allaah’s Peace, Mercy, and Blessings be upon you!) To proceed:
Our beloved brother! We have received your message dated 19/12/1388 A.H. May Allaah guide you to His Straight Path. You inquire about the ruling on a man who married a woman but he was breastfed by her mother while the latter was married to a man other than the father of the former. Your inquiry about the ruling on such an issue is known to us.
Answer: This marriage is null and void since the man referred to in the question is a brother to the woman he got married to since he had been breastfed from her mother. This prohibition is quite evident in the Qur’aan, Sunnah and Ijmaa` (consensus) of Muslim scholars provided that her mother had breastfed him five times while he was in the first two years… read more here.