Are the sons of the maternal uncle considered maharim to the daughters of the sister if their mother breastfed her sister's son, and her sister breastfed her other son?

Fosterage establishes the same prohibitions as lineage. The suckling child becomes a son to the wet nurse and her husband. This applies only to the suckling child, not to his siblings.

Therefore, your maternal cousin's son whom your mother breastfed is considered your brother and the brother of all your siblings through fosterage. This does not include his other siblings who were not nursed by your mother.

And your brother whom your maternal uncle's wife breastfed is considered a foster brother to all your maternal uncle's children. As for you and your siblings, there is no prohibition between you and your other maternal cousins if you were not nursed by their mother.

Consequently, the women of your family must observe hijab in front of unrelated maternal cousins, and your maternal cousins' daughters must observe hijab in front of your unrelated brothers. It should be noted that the fosterage that establishes prohibition is five full, satisfying breastfeeds.