Is it permissible to marry one's paternal aunt's daughter, knowing that the questioner and her paternal aunt's son were both suckled by the grandfather's second wife (who is neither the mother of the father nor the paternal aunt), and the number of times they were suckled is not remembered?

If you suckled from the man from whom your paternal cousin suckled, then it is not permissible for you to marry her, even if you doubt the number of sucklings. This is based on the hadith of Uqbah ibn Al-Harith, where the Prophet (peace be upon him) commanded him to separate from his wife after it was said that they both suckled from the same woman. He said to him: "How, when it has been said?" and in another narration: "Leave her." And because some scholars hold that prohibitory kinship is established by a single suckling, it is safer to avoid marrying her to escape disagreement and to act upon the hadith: "Leave that which makes you doubt for that which does not make you doubt."