Should i tell my parents that my sister has a girlfriend?

I assumed my sister was into women for a while but the other day i walked in on her and another girl intimately. She was embarrassed and asked me not to tell but i have been seeing her a lot with this girl and shes been staying out late and acting a bit rebellious. I'm worried about her and i feel that maybe if our parents know a little bit about who she is it'll help. Should i tell my parents?