time to call child protective services, that is sexual abuse whether it is intended to be or not, he is practically an adult touching a naked little girl. Call the cops and if your mother thinks its ok she is just as guilty as the pedophile baby sitter. Call the cops, if you don't then your just as bad as the babysitter and your mother, and legally just as guilty, knowing something like that was going on and doing nothing to stop it
oh and by the way some sister you are, you would rather go out with your friends and let your sister be sexually abused. I can see where your priorities are but hey i give you props for trying, you messed up at the end when you said you would baby sit yourself but your always busy with your friends, just drop that part and you would be a great troll. =)