Please help!!!!

I don't know what to do!!!!!
My sister (28) has a new boyfriend who I think is abusive. I'm almost 100% sure he is. She has bruises and marks all over her back/arms and some on her legs, but I really don't know what to do about it. Please, what do I do? Should I tell my parents? I'm only 19
EDIT: the bruises are dark. So dark that there's no way they could be from play fighting or rough sex. These bruises and markings look like the shape of fingers and slaps.