man is a Mahram (an unmarriageable relative) to the daughters of his maternal nieces and nephews and their descendants, because he is a maternal uncle to them and they do not have to be veiled in front of him, as Allah (Exalted be He) says:
Forbidden to you (for marriage) are: your mothers
...your brother’s daughters, your sister’s daughters
This also includes their descendant daughters too. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.