Is it permissible for a young man to marry his maternal cousin if his mother swears that her mother (the young man's maternal grandmother) breastfed him, while the grandmother swears that she did not breastfeed him, knowing that proof of breastfeeding would make the maternal cousin the young man's sister through fosterage?

An affirmative testimony of breastfeeding takes precedence over a negative one, as the former includes additional knowledge. Breastfeeding is established by the testimony of one righteous woman, if she testifies to five full, satisfying breastfeeds. Based on this, if the young man's righteous mother testifies that he was breastfed by his grandmother five full, satisfying breastfeeds, then he becomes the maternal uncle of his maternal cousin's daughter, and she is forbidden to him. There is no sin or expiation for one who swore based on what they predominantly believed, as this falls under a 'laghw' (unintentional) oath.