12-Year Old Girl Wearing Makeup?

My Niece Is A EXTREMELY beautiful girl! She is gorgeous! My sister(her Mom) wont let her wear makeup. She is in 7th grade. She told me she didnt want to wear eyeliner, just mascara and blush. My sister is really strict with my niece! If she was my daughter Id let her. Shes not doing it for slu tty intentions either. I Think my sister is overreacting? Dont You? She Told Her Not until highschool! What Should we do to convince her?? Thanks!(: