Is it permissible for my daughter to uncover before my father's sister's sons, after I breastfed my paternal half-brother?

Your suckling your brother makes him a foster-brother to your children, and spreads the prohibition (of marriage) between your brother's children and your daughter. Thus, it is permissible for them to look at her and be in seclusion with her. As for your sister's children, they are non-mahram to your daughter, so it is not permissible for her to appear before them except in the prescribed Islamic dress.