My sister is a pervert...?

My sis came to live with me on february and she is 2 or 3 years older. Anyway my niece, she is kind of abnormal because her had was drugged so shes a bit crazy and dumb but anyway, she is 8,acts like shes 5 and my sis is 14. My sister has been a perv but yesterday she was being dirty. First she gets all close with my niece and all n they hugg real hard and all. Then at night they went to sleep together, my sis tried to make less space on the bed so theyll be closer and then they were playin around but in a dirty way. My niece was tryna look at my sis and all and pullin her shirt and even though my sis tried stopping her i bet she still liked it. And my sis did it back they were also tryin to do it down there. I didnt say anything, i even cried cause i dont want my neice to be lik her. I also dont feel like tellin my parents not sure why, i try to talk to my neice but she dlnt understand me. And i aint the type to tell my sis not to do that but is there a way to stop this without havin to say anything to parents or sister?