Is the sister's daughter unlawful for the questioner's son to marry, given that the questioner breastfed her a few unsaturated times – she doesn't recall the exact number, whether four or five – on the fifth day of her birth, where the milk was scant and did not satiate the child, and all of this occurred within five hours?

According to the preponderant opinion, the prohibition (of marriage due to fosterage) is established by five full, satisfying sucklings. The definition of a suckling is subject to common custom. Its criterion is that the child suckles from the breast until he voluntarily leaves it. If this occurs five times, then the prohibitive fosterage is established. However, if there is doubt about the number of sucklings—whether they are four or five—the prohibition is not established, because the default is non-existence until it is proven with certainty. Based on this, if there is doubt about the number of sucklings or about the legal validity of each suckling, the prohibition is not established, and it is permissible for your son to marry his maternal cousin.