What should I do

What should I do I got in an argument with an old friend and she threatens to beat my stuttering ass sister up. My sister is 15 and she is 19. I’m so pissed. I ended a friendship with my other friend because she is still friends with the girl who made the comments. Every time I think about it I get so angry. I want to cut off anyone that knows her or knows her friends I’m so upset. Is it unreasonable for me to cut off anyone that hangs around people she is friends with.