I know sitting back and hoping is the easiest thing to do in this situation, but IT IS NOT THE WISEST! As other people have said, you can get into SERIOUS shit for this. I really think you should a) tell someone or b) just GET RID OF THE PHONE.
CHRIST I'm sorry but your sister sounds like a CUNT. Send her to boarding school >;c
I would either A. Tell your mom that your sister had THIRTEEN-year-old penis's on the phone or B. Place the phone in front of a car and smash it.
I don't think it'd be good as black mail, seeing as your sister could say 'BUT YOU PUT THEM ON THERE.' THOUGH. Though. All pictures have information dates on them. If you look at the info, you can see when the picture was stored and you can just say 'But this picture was on the phone before I got it!' and show proof.