It is most definitely NOT your fault. Both you and your sister were the victims in this crime. Does it help to know that your sister validates your story?. So many people struggle with "did I make this up? / am I just trying to get attention / am I a bad person who is out to get [abusive person]"

I'm sorry your parents reacted the way they did. There's no such thing as a little sexual assault, just as there's no such thing as being a little pregnant. What was done to you was traumatic. I hope you are getting help to deal with this (and that your sister is too)

(((safe hugs))))