Is it permissible for me to marry my maternal cousin, given the doubts about whether my fiancée's mother was suckled by the same woman who suckled me, despite the wet nurse's affirmation and my grandmother's denial of the matter?

The preponderant opinion among scholars is the permissibility of accepting the testimony of a wet nurse alone regarding her having suckled a person, because no specific benefit accrues to her from it. This is the madhhab (definition) of the Hanbalis. They cite as evidence the agreed-upon hadith (definition) of 'Uqbah ibn Al-Harith, where the Prophet, peace and blessings be upon him, separated him from his wife based on the statement of a woman that she had suckled both of them. Based on this, it is not permissible to marry this girl, as she is the suitor's sister through fosterage.