Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

 

Your brother is a brother to the daughter of his milk-mother or the woman who breastfed him. Therefore, he is not allowed to marry her since he is her mahram.

 

We read in the Qur’an:

 

“Forbidden to you are your mothers, and your daughters, and your sisters, and your aunts paternal and maternal, and a brother’s daughters, and a sister’s daughters; and your milk-mothers, and your milk-sisters; and the mothers of your wives; and your step-daughters – who are your foster children – born of your wives with whom you have consummated your marriage; but if you have not consummated your marriage, you will incur no sin [by marrying their daughters]; and [forbidden to you are] the spouses of the sons who have sprung from your loins; and [you are forbidden] to have two sisters [as your wives] at one and the same time – but what is past is past: for, behold, God is indeed much-forgiving, a dispenser of grace.” (An-NIsaa’ 4:23)

 

Almighty Allah knows best.