I'm so glad that the internet didn't exist until I was in my 20s. The amount of damage that you can do to yourself or to others in typical 'daily life'was so much less than what you can do now, when you're online.

I'd concentrate less on restricting her access to the internet, and more on having good conversations surrounding safety, self-image, sexuality, etc. You aren't going to be able to restrict her access to online life in any meaningful way, without her thinking that you're just out to get her.

It sounds like she might be a bit 'teksavvy' for her age, but not at all 'life savvy'. If she hasn't had comprehensive sex ed in school that encompasses health as well as safe/healthy choices, start talking to her about that now. Scarleteen is a very useful site- not only about the physicality and information about sex, but about making good choices. If you're going to make sure that she's educated about choices and her body and her sexual choices, you need to be willing to have explicit conversations with her. She's probably already seen anal sex videos, and gay/lesbian porn. It wouldn't be a huge surprise if she's already run into Harry Potter slash fic, tentacle porn, my little pony fandom etc.

I'd try to ensure that she sees you as someone that she can talk to, now, and in the future. She's making bad decisions now, and that is heartbreaking, but you trying to control the websites that she visits or keep her safe isn't going to do much good. Let your sister know that she can talk to you, and that you are her safe space. Try not to judge her- my kids are almost 20, and it still freaks me out a bit that I had to explain about anal sex when they were in grade 4. Kids now have challenges that we didn't, and I think that the best thing that you can do is to keep a conversation going.