Hello~
I think I can help ease your fear (a bit). Hopefully.
I started watched porn at 8-9 years old (im a girl) and
It was a lot of BDSM, non-consensual stuff. At that age I even fantasizes about being kidnapped or raped, which sounds really bad. I suffered a lot of abuse as a kid though, and had a very emotionally unattached father which I think was the kicker. You should examine your sisters relationship with her dad in particular (father daughter relationships affect sexuality heavily). But she may just be sexually perverted, which isn't too abnormal, and doesn't mean she was molested. Most women I've met who were molested actually don't want anything to do with sex in any way, but that doesn't mean it never happened. The best thing you can do is either ask your sister directly, like taking her to get ice cream and sitting down to chat. Or, if it really worried you, bring it up to your parents. She could just be an over sheltered child with pent up sexual energy that manifests itself in odd ways. Either way, it sounds like she's just a kid exploring some odd desires, and the best thing you can do is make sure she understands her own sexuality, self worth, and what it is she's looking at. BDSM porn (hardcore or not) and even non-consensual stuff is not a sign of any serious mental problems, because BDSM plays on carnal/primal sexual needs and interests. It's not a scary, abusive thing like one would believe, and hopefully she will understand that her desires are most likely a form of possessive, powerful love and lust with a need for control and vulnerability, and she won't grow up to gravitate towards abusive relationships because of it. Anywho, good luck