My 15 year old sister is sexually active?

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I'm a 16 year old girl and I've always known that my younger sister has been quite slutty (when she was 13 she would go on a night out and kiss up to six random guys). but now she has become so much worse. last year I found out that she had been sending naked pictures to a number of boys whom she hardly knew through BBM (when she was fourteen). she has no common sense so when horny teenage boys ask for a pic she just gives it to them, anyone. I confronted her about this and told her she needed to stop and she acted like she didn't know what I was talking about. recently I found out that while she was going out with her boyfriend last year (aged 14) that they "made love" as she put it (i read her facebook messages) and she also gave him and his friend numerous pictures and blowjobs and he also licked her out. she is now 15 and I just found out that she also had sex behind the health centre at the local Easter fair with a boy she just met. she's on the pill for her skin problems so she won't get pregnant, but I'm worried she might pick up stds and also about her giving herself a bad reputation and also me as her sister. she is obviously the favourite child and my mum thinks she's so innocent and is always comparing us, and I hate it cos all I've ever done is kiss boys that I know and I am not a slut. I just don't know what to do, any help is much appreciated.