What is the ruling on a man marrying his paternal aunt's daughter, when his paternal aunt breastfed him as a child? And is his breastfeeding by her considered a prohibitor of marriage, or is it permissible for him to marry her?

It is preferable not to marry a girl whom your mother definitively states she breastfed you, especially since some scholars believe that breastfeeding establishes prohibition even if the milk was yellow. You should also not treat her as a محرم (unmarriageable kin) to block the door to temptation and to err on the side of caution. Allah may compensate both of you with something better. Allah Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not." (Al-Baqarah: 216).