My sister is ruining the family?

My 12 year old sister is ruining the family. She used to be nice, but for the past few years she's turned into one of the meanest people I know. She's extremely cold and when anyone tries to hug her or just touch her, she'll scream, "Don't touch me, you perv!" At family get-togethers, she just sits on the couch, watching TV, with an evil look on her face. She never helps watch the toddlers or do any work in general. She hates everything, so whenever we go anywhere she ruins the mood by complaining and sulking the entire time. She's also really crappy to me and our 8 year old sister, who has a developmental delay. She swears at both of us constantly and has called the 8 year old a freak multiple times. My parents have taken away her phone and grounded her for it, but all she says is, "I don't care. " I can't even go with her anywhere because I know how she's gonna act. I really can't deal with this much longer, and my parents are getting sick of it too. How can I get her to act like a somewhat decent person?