Assalaamu alaykum. My mother breastfed my oldest sister and this boy, Ally, together. So I believe they are brother and sister. I was born many many years after. Ally got married and has a daughter. I would like to marry Ally's daughter. Am I allowed to marry Ally's daughter given that her father shared milk with my sister? Thank you.

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 that man named Ali breastfed from your mother along with your eldest sister – as you mentioned – then by this he becomes your brother through breastfeeding. Therefore, it is not permissible for you to marry his daughter because she is the daughter of your brother from breastfeeding. The fact that you did not breastfeed along with him is of no effect Islamically; what is important is that he breastfed from your mother.

It should be noted that the breastfeeding that makes it forbidden for one to marry someone else who breastfed from the same woman is breastfeeding five times one’s full in the first two years of age according to the preponderant opinion, which is also the view we adopt here in Islamweb.

For more benefit, please refer to fatwa 83788.

Allaah knows best.