Praise be to Allaah.
If breastfeeding is proven to have taken place, and he was breastfed fully five or more times within the first two years of life, then your brother is a son to your maternal uncle through breastfeeding, and he is a son of your uncle’s wife who breastfed him. Their children are his brothers, and your maternal uncle’s brothers are his paternal uncles and his sisters are his paternal aunts. The brothers of the woman who breastfed him are his maternal uncles and her sisters are his maternal aunts, because the Prophet (peace and blessings of Allaah be upon him) said: “What becomes mahram (forbidden for marriage) through breastfeeding is that which become mahram through blood ties.” Saheeh – agreed upon.
But you have no connection with the breastfeeding mentioned, and it is not permissible for you or your sisters to uncover in front of the sons of your maternal uncle on the grounds that your brother was breastfed by the wife of your maternal uncle, because they are not mahrams for you.
May Allaah help us all to understand His religion and to adhere to it.