Am I a Mahram (unmarriageable relative) for the wives of my nephews and grandsons? Similarly, am I a Mahram for my grandnieces?
You are not a Mahram for the wives of your nephews, (Part No. 17; Page No. 442) but you are a Mahram for your grandson's wife, as she is mentioned in the Ayah (Qur'anic verse):
...the wives of your sons
Also, you are a Mahram for your grandnieces, as you are their father or mother's uncle. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.