Is it permissible for a young man to marry his maternal aunt's daughter if the girl's mother suspects that she breastfed the girl along with the young man's deceased sister, without certainty about the number of feedings or their sufficiency?

If the girl's mother is in doubt about the prospective suitor's sister having been suckled by her, then this doubt has no effect. And if the prospective suitor's mother is in doubt about her having suckled his fiancée, then this also has no effect, due to the doubt regarding its occurrence and number, and the original presumption is its non-occurrence. However, it is more prudent to forgo marriage in this situation, as a precaution.