What is the ruling on marrying the daughter of a maternal aunt whom one's mother breastfed fewer than five times, and is she considered a sister through breastfeeding?

The suckling that establishes mahramiyya (prohibition of marriage due to kinship) is five sucklings within two years [of age]. Anything less than that does not prohibit marriage. However, it is more cautious not to marry the girl who suckled from your mother, even if it was less than five sucklings. This is due to a jurisprudential difference of opinion among scholars regarding the number of sucklings that establish prohibition. Some scholars hold that both little and much suckling prohibit, while others believe that less than five sucklings does not affect the prohibition.

Thus, the matter is subject to scholarly dispute. The more cautious and conscience-absolving approach is not to marry this girl, especially since she is one of your relatives (your maternal uncle's daughter). Some jurists have recommended marrying non-relatives to avoid severing ties of kinship in the event of divorce.