Okay, while it sucks that you saw that, and is disturbing and gross that she used your toothbrush to do that... a lot of girls start puberty as early as eight or nine years old nowadays, which means that by ages twelve to thirteen, most of them have developed a sex drive on- at least- a hormonal level. Better for her to use a toothbrush (though hopefully not yours anymore) than to end up in some situation with a boy because she doesn't understand the risks. If your parents won't talk to her about it, please do her a HUGE favor- though she may resent you for it now- and explain things to her. Hormones, pregnancy, contraceptives, STDs, safe sex, CONSENT- everything that your parents haven't and/or won't. Particularly explain that using a toothbrush that way is unsanitary for both her and the owner of said toothbrush, and if she really feels like she needs one for that, to get it and ONLY use it for that. Of course, all of this can wait until you are done vomiting. (Seriously though, make sure she understands what consent is and that she needs to tell someone if someone does or tries to do something sexual to her without it. Too many girls and boys get taken advantage of by older teenagers and even adults when they first start developing a sex drive. Worse, far too many of those that are attacked that way don't understand that they aren't the one at fault and that they didn't deserve it, regardless of what any backwards snobby idiot stuck in the 1800's may say.)

Someone needs to have a bird and the batteries talk with her.