What do i do with my best friendz sister?

ok if this takes long im srry!!! let me tell u a bit bout me i can be pretty mean ,when ppl tak bout me i don't care but when it comes to my frndz u are so crossing the line. i have 2 best friends a girl and a boy ,we'll call them jane and idk bob lol. and everytime i talk to jane she complains about her sister and how mean she is and well i didn't know the girl so i didn't care .well i came home for christmas a few days a go and today me and a good friend went to visit bob and jane's sister and a friend of herz was there so we walk in and they go up to my friend and they're like y dd u come? and then she's like it's my boyfriends house we're just saying hi and we're gonna leave . then we say and we stand outside to wait for our ride and we're being stupid then i needed some tissue to blow my nose and i go in and hear jane's sister and her friend making fun of us and then i go outside and im quiet coz what they were saying about my friend was really mean i didn't listen to what they were saying about me because i left right away and then my friend was like whatz wrong i didn't want to tell her but shez really sensitive and she started thinkin it was her fault bcoz she dd something and i said no and then she though bob was gonna break up with her so i told her but i didn't tel her what they said i just told her they were making fun of us and she's like i'm not surprised they're really mean and she got really upset and said thank you for telling me because i like to know these things and i feel really bad right now .now tht my nove is over is it wrong for me to get revenge from my best friendz sister because she hurt my other friend?