A mother's paternal uncle as Mahram to her daughters
If my niece has a daughter, is it permissible for me to treat her daughter as I treat her mother or not?
You become a Mahram (unmarriageable relative) to her, as you are her mother’s paternal uncle, and therefore her uncle as well. The same applies to your niece’s granddaughters and their descendants. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.