YES! YOU SHOULD DEFIANTLY TELL! Its VERY wrong that she did that, she probably knows that, but still, she should know what consciousnesses there are, what if she gets pregnant? I'm young also but I was taught better to do something like that. "she seemed genuinly upset and promised never to do it again until she was legal and not just with a random." maybe a lie, but who knows I'd take the risk of telling her mother or father, or in your case my cousin about what her daughter did. And if she was sending pictures you'd never know what else shed been doing. Its a risk of the girls trust but its better then finding out what might happen in a few years or so. Then its up to what her parents think(Hopefully they make the right choice). Its just my opinion, but I'd rather share something very serious like this then leaving it alone and not sharing. You should only do, what you KNOW, you wont regret tomorrow. If I typed that right... :)