Is the cousin (daughter of the maternal aunt) considered unmarriageable if the maternal aunt breastfed the maternal uncle, knowing that the cousin is younger than the maternal uncle who was suckled by her mother?

Your aunt's breastfeeding of your maternal uncle spreads حرمة (prohibition of marriage) between your maternal uncle and your maternal aunt's daughters due to kinship and suckling. This makes your maternal aunt's daughters paternal aunts to your maternal uncle's children through suckling. As for you, there is no prohibition for you to marry any of them, unless there is another reason for prohibition.