To me it is very simple: if it is not CONSENSUAL, then it is a rape.
Or to put it bluntly: if it is not "YES" then it's "NO".
Your sister has more responsibility than the rapist. We all know what rapists do, but we don't expect our sisters to push us into their arms by force. I hope you do not have any relationship with her; if you do, you are putting yourself in danger. She is jealous of you, hates you, and obviously wishes you all the worst things in life. Stay well away from her (and all her friends); just forget that she exists and treat her like a stranger.