Hey, Reddit. i need help dealing with a girl who is hurting my sister. any ideas?

so my sister has this friend and up until this year she was very nice, but she asked us for alot of rides and would be a little rude about my family and our current economic situation. In about april she called out my sister and said that she wouldnt have talked to her if she had known how annoying she would end up being and how she doesnt want to see her in college and she then insults my family by saying that my sister only used her for her wealth infront of about 30 kids. Alittle while later she tweets some awful things about my sister(calling her fat and ms. piggys body double). she also reported my other sister whom she was friends with aswell to the police for something she didnt do....and lastly, today she yelled at my sister while at a grad party for a mutual friend of theirs.

this girl is crazy and I dont know what to do. im scared for my sisters and i dont want her doing this to me, being that we will be in the same show this summer.

can anyone advise me on what to do here?