Should i tell my mom?

I have seen something from my sisters boyfriend which my sister knows nothing about (its about betrayal and i am sure about it) and my sister is still young but insists on marrying this guy which i disagree with. there is nothing i can do but i know my mom can interfere and save her from marrying him but not sure if this is the right thing to do?to tell my mother about my sisters personal problems.Should i keep quite because its not my business or since i should do something about it as her sister?