[The Ruling on a Grandmother Offering Her Breast to Soothe an Infant 2]

Answer—and Allah grants success: If the old woman’s breasts are dry and nothing comes out, that is not considered breastfeeding, and it is not forbidden for one of the two to marry the other. But if, when she played with the two and calmed them in that way, there was any fluid issuing from her breasts, then that counts as breastfeeding, and it is forbidden for one to marry the other.
In sum: if anything from the woman’s breasts entered the stomach of each of the two children, then the old woman thereby becomes the mother of both, and the two become milk-siblings. If it entered the stomach of one but not the other, they do not become siblings and one may marry the other—unless the old woman is their grandmother (i.e., both are her grandchildren). In that case, nursing even one of the two prevents their marrying each other, because the one nursed would then become an uncle or an aunt.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1