Are the daughters of the questioner considered mahrams to the supposed maternal uncle, who was nursed by the mother of the questioner's wife, such that they may unveil in front of him?

If the man mentioned was nursed by your wife's mother or her stepmother five times or more within his first two years, then he becomes a maternal uncle to your daughters through nursing. It is permissible for them to unveil in front of him and to be alone with him, due to the Prophet (peace and blessings be upon him) saying: "What is prohibited by lineage is also prohibited by suckling."