Is it permissible for the suitor to marry the girl, even though her father was nursed by the suitor's eldest sister an unknown number of times?

Lactation establishes the same prohibitory kinship (mahramiyyah) as lineage. So, whoever suckles from a woman becomes her son and the son of her husband, the owner of the milk. If the suitor's father suckled from the milk of the suitor's father (from another wife) along with the suitor's elder sister, he would become her brother.

The lactation that spreads prohibitory kinship and establishes prohibition must consist of five full sucklings (رضعات مشبعات), according to the more preponderant opinion. If there is doubt about the number of sucklings, then the lactation is not established due to the presence of doubt, because the default is that lactation is not established except with certainty.

If lactation is not proven, then marriage to the suitor is permissible. However, it is more cautious to avoid it, out of consideration for those who hold the view that even a single suckling is to be taken into account.