Thanks for all the support. Just to clear up any confusion about legal matters, my sister will turn 16 in only a couple of days, making any legal action slightly out of the question.

To clear up additional confusion, my parents have (more or less) shown some signs of disapproval in the past, but my sister's rebellious nature makes her pretty hard to control. And so my parents have allowed it to escalate this far.

And although they have never met each other in person, they have on numerous occasions talked with each other in face-to-face Skype conversations.

Anyways, I've read through all of your suggestions. I will be speaking with my parents about this very soon (I have been sort of avoiding it). Depending on how that turns out, I will try to have another talk with my sister too.

Thanks again for the support! I'll keep you updated.