Sorry to hear that mate
I think your sister sees this as normal because I remember one time when I was like 8 I jammed my "area" into the couch cushions because it felt good
Years later,when the world was open and all secrets were revealed,I realized what I did
To this day,nobody except for me knows
So,basically what I'm saying is this:
I understand what she did was weird,and I also understand it could be frightening,but I am also saying that we all did dumb stuff when we were younger and you should try to calm down and try to forget