Being a Mahram to one’s foster sister’s daughter

In the queried scenario, Ahmed and Fatima become (milk) brother and sister to each other, due to drinking milk from the same mother. Fatima’s daughter will therefore be considered as Ahmed’s niece (foster-sister’s daughter), making Ahmed a Mahram to her.

There is a poem that explains the principle of breastfeeding which is as follows:

The breastfeeder’s family become Mahram to the breastfed child,

The breastfeeder becomes the mother; her husband becomes the father; her sister becomes the Khaala (maternal aunt) and her children become brothers and sisters.

As for the breastfed child, the Hurmat is restricted up to the one’s children and spouses. If the breastfed child was a male, then the breastfeeder’s husband cannot marry his wife, and if it was a female, then the breastfeeder cannot marry the breastfed child’s husband. [Re:- Fataawa Darul Uloom Zakariyya Pg.396 Vol.4]