Is the daughter of the questioner's sister—whose sister breastfed her brother from the father—considered her sister by suckling, and does she become an unmarriageable maternal aunt to her children? What is the ruling on interaction between the questioner's children and their relatives after knowing the ruling on suckling?

The sister's daughter is not considered a foster sister merely because the sister breastfed the brother. Therefore, your children are not unmarriageable kin to your sister's daughter. As for your brother, whom your sister breastfed, he has become a foster brother to all of your sister's sons and daughters, as well as to the sons and daughters of her husband, due to whom the milk originated. As for the interaction of your brothers and sisters with your sons and daughters, they treat them as unmarriageable kin, based on the Almighty's saying: "and daughters of brothers and daughters of sisters, and your mothers who nursed you."