Is it permissible for a woman to marry her fiancé, her maternal aunt's son, despite the aunt's claim that she breastfed her during infancy, given the doubt about the occurrence of the forbidden suckling?

For fosterage to be prohibited, there must be five separate sucklings within the two years of infancy. A suckling is defined as the infant latching onto the breast until he leaves it willingly. This is supported by the narration of Aisha (may Allah be pleased with her): "Among what was revealed from the Qur'an were ten known sucklings that make marriage unlawful, then they were abrogated by five known sucklings." The Malikis, Abu Hanifa, and Ibn Taymiyyah differed regarding the period of prohibitive suckling, but the preponderant opinion is that of the majority, which stipulates that the suckling must occur within the first two years, based on Quranic verses and Prophetic traditions. Therefore, if your fiancé's mother was suckled five times by your maternal aunt within her first two years, your fiancé becomes your foster brother, and marriage to him is impermissible. However, doubt does not establish prohibition.