Caught nephews with moms vibrator :/?

Ok, moral dilemma. Sent the kids to bed and decided they could sleep in their moms room, I heard loud screaming so I went to check out what they were doing and the older one (9) had found their moms vibrator and had put it in the 5 year olds pants. He was acting suspicious and tried locking the door several times earlier in the day. I grilled him about it after flipping out on both of them from the initial shock and he told me he had gotten into it a few times before. I feel almost profoundly sickened by this, however I also feel like I will be blamed for not watching them closely enough. How should I explain this to my sister? The younger one went so far as to tell me he was screaming because it "felt funny" on his , to which I just explained it was a personal item of his mothers and was used for something entirely different than that... I don't even know if I want to watch these kids anymore after this. Thanks all.