Touchy situation. Need SERIOUS advice

I'm concerned about my daughters arrival for the simple fact that my MIL and SIL want to babysit my baby! I don't want to even imagine leaving my baby with them alone. 
The reason I'm concerned is because my SiL told me she was molested by her step dad and her mom (mil) never did anything about it... She's still with her step dad.. (This happened years ago) I feel like if something was to happen my MiL would keep quiet too. Nope!!! I won't allow it!!! Am I being too over protective?!?!