Are the children of the elder sister maternal uncles to the daughters of the questioner, whom her maternal uncle's wife breastfed, and she [the maternal uncle's wife] also breastfed the daughter of her own elder sister?

Your sister's daughter, who was nursed with you, is considered your sister through breastfeeding, and an aunt to your sons and daughters. Her brothers and sisters are not unmarriageable (mahram) to your sons and daughters, because the breastfeeding did not affect them.