Is marriage to the son of a maternal uncle forbidden if his sister was nursed with the sister of the deceased fiancée?

It is permissible for you to marry your paternal uncle's son, as long as you were not breastfed by his mother, and there is no other reason for fosterage that would create a prohibited degree of relationship between you. The marriage is not forbidden due to his brother being breastfed by your mother alongside your younger sister, whether she is alive or deceased.