Hi welcome,
Until your sister matures the only thing you can do is be the bigger person and ignore her. It isn't easy to do at a young age but if you master it now it's so much easier when older.
With social media use the block feature when you need to. If others believe her description of you then those people aren't the type of people you need in your life.
You seem to be a kind and considerate person, hold onto those qualities and try to accept that in life, you can't fix everything especially attitudes regardless of family or friends. However have hope that one day she will be remorseful and want to get close, then could be the time to test the waters to see if she has matured.
It's great you wrote in, that shows good decision making. I hope I've helped.
TonyWK
