What is the effect of the breastfeeding of the questioner’s eldest sister by her youngest maternal aunt on the kinship and lineage of the questioner and her siblings with their maternal uncles and aunts, and is it permissible for the eldest maternal cousin to marry one of the questioner’s sisters?

If your youngest maternal aunt was breastfed by your mother five known feedings, then she is your sister through fosterage, and the same rulings apply to her as apply to your sisters by blood, because fosterage prohibits what consanguinity prohibits. As for your relationship with your other maternal uncles and aunts, it remains as per the original ruling: their sons may marry your sisters, and your brothers may marry their daughters.