Are my maternal uncle's children, whom my mother breastfed, considered my siblings through nursing, and does the same ruling apply to their siblings whom my mother did not breastfeed?

A brother through breastfeeding is anyone who was breastfed by the woman who breastfed you, whether she breastfed him before you or after you, and whether this woman was your mother or someone else. So, if a woman breastfed you and then breastfed other children, they become your siblings through breastfeeding. This is by scholarly consensus, and its basis is the Almighty's saying: "And your sisters through suckling." Accordingly, your uncle's children whom your mother breastfed are your siblings through breastfeeding.