Is it permissible for me to marry the daughter of my maternal aunt, who nursed with my sister, knowing that my mother does not clearly remember having nursed her, and her mother denies it?

If it is established that your female cousin (daughter of your maternal aunt) was suckled by your mother five times or more within the first two years of her life—even if attested by a single woman—then marriage to her is not permissible. This is because she would be your sister through breastfeeding, based on the Almighty’s saying: "And your sisters from suckling" (Quran 4:23), and the Hadith (definition): "What is forbidden by lineage is also forbidden by suckling." However, if breastfeeding is not established, or if it was less than five sucklings, then there is no impediment to marrying her, although it is better to avoid it.