Should i go to it or not ?

you should for sure go. you should have fun...dont worry about the mean girl-be nice to her and try to make friends with her. maybe she soesnt know you that well. and if she isnt being nice about it...then oh well. just dont interfere with her. she will be fine. and as for the birthday girl...just show her that you dont hate her or anything and maybe you guys will end up as way close friends. :D