My sister is being a pervert..?

My sis is 14 and came to live with me since february. My niece is 8(acts like 5) shes kind of abnormal ish i think because her rl dad was drugged and all. Anyway my niece came to stay with me a week ago for a while. And yesterday something went awkward and i already know my sis is a pervert but this one seems wrong. My sister was being all over her and stuff and been huggin her so tight but at night they were playin around in the bed. My niece was like tryin to see my sisters breast and oullin her shirt and all. Even though my sis was stoppin her a lil she probably still like it plus she even did it back and tried to touch down there also. But i believe its my sis fault cause my niece was never like that. Then when they sleep my sis was almost on top of her huggin her so close. And today few mins ago my sis was goin all close to her again and hugged her that made them look like gf and bf. And she keeps on carryin my niece when she can walk herself. I never said anythin i dont really want to but if i had to im not sure what to say. I dont even want to tell my parents..