Is it permissible for a male cousin to marry his female cousin if his mother breastfed his female cousin (the prospective wife) along with another one of her sons who is not the husband himself, and the husband claims he was not breastfed naturally?

If your mother was suckled by her paternal aunt five separate times before the age of two, then her paternal aunt becomes her mother through suckling, and the children of her paternal aunt become her siblings through suckling. Thus, it is not permissible for your mother to marry any of them. Prohibition through suckling includes all the children of the wet nurse. Scholars have differed regarding the number of sucklings that constitute prohibition; the adopted view among us is five sucklings, while others hold that prohibition occurs even with a single suckling. The more cautious approach is not to marry her paternal aunt's son.