Is it permissible to marry the daughter of my paternal uncle, who is the eldest son of my maternal aunt who breastfed me, and is there a prohibition between me and the daughters of her other sons, given that my mother breastfed my brother two or three times, and I was breastfed by my maternal aunt two or three times?

If your mother breastfeeds your male cousin (your maternal aunt's son) five full, satisfying breastfeeds, then this uncle becomes your brother through suckling (rada'a), and his daughter is forbidden to you because she is your brother's daughter. However, if your maternal aunt breastfeeds your sisters only three times, they do not become her daughters, nor sisters to her children, because the prohibition (tahrim) is not established except with five full, satisfying breastfeeds, which is the legal opinion we adopt. Doubt concerning the number of breastfeeds does not establish prohibition. It is more cautious not to marry the daughter of the elder paternal uncle if there is doubt regarding the number of breastfeeds.