Is it valid that my brother's suckling from my paternal uncle's wife and my paternal uncle's daughter's suckling from my mother is considered reciprocal suckling, and is it permissible for me to uncover before my paternal cousins (my husband's brothers) based on a fatwa that states it is reciprocal suckling because "the milk is the father's milk"?

Your brother who was suckled by your paternal uncle's wife becomes a brother only to your paternal uncle's children, and not to his siblings who were not suckled by her. Similarly, your paternal uncle's daughter who was suckled by your mother becomes a sister to you, your brothers, and your sisters, but not to her siblings who were not suckled by your mother. This is because only the suckling child is considered the child of the milk-giver and her husband. Your paternal uncle's daughter who is married to your husband's father is not your sister through suckling merely because her sister was suckled by your mother, or because your brother was suckled by his paternal uncle's wife. Therefore, her children are not your mahrams, and it is not permissible for you to uncover in their presence unless there is another reason for mahramiyya. The suckling that prohibits is five satisfying sucklings.