What is the ruling concerning my brother and I marrying our female cousins from our maternal aunt, given that my maternal grandmother breastfed my brother, and my mother and aunt breastfed her younger sister?

If a brother is suckled by his grandmother five known sucklings before the age of two, he becomes a brother to his mother and maternal aunts. Consequently, it is impermissible for him to marry his maternal aunt's daughter because he has become her maternal uncle through suckling.

And if the youngest maternal aunt is suckled by the mother five known sucklings within two years, she becomes a sister to the questioner and his brother, and they become maternal uncles to her daughters. Therefore, it is impermissible to marry the daughters of this maternal aunt.

Prohibitive suckling is established by five known sucklings within two years, and it entails the prohibition of marriage to relatives by suckling, just as is the case with lineage.