What is the ruling on a father uncovering before his foster daughters, knowing that the children nursed from the same woman and thus became siblings?

Lactation establishes what lineage establishes, for the Prophet, peace and blessings be upon him, said: "What is prohibited by lineage is prohibited by lactation." So, whoever is suckled by a woman becomes her child and her husband's child. If the wife of each of the two men suckles the children of the other, their children become siblings through lactation, and each of them becomes a father through lactation to the children of the other. Therefore, it is permissible for the daughters of each of them to uncover before their father's friend (their father through lactation) what is permissible to uncover before mahrams. It is stipulated for prohibitive lactation that there be five full sucklings, according to the preponderant opinion.