Answer
(Fatwa: 658/658=M/1430)
You cannot marry your brother with the cousin sister (the daughter of maternal aunt); since in the abovementioned case, by drinking the milk of maternal grandmother, your brother and cousin sister became foster maternal uncle and niece. As it is unlawful to marry real niece, it is unlawful to marry foster niece.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband