Although it must be annoying for her to suddenly decide on things, especially with cosplaying the same characters you want to do. But remind yourself that you both don't own the characters nor can change each others decisions. Do what YOU want to do, and completely ignore her tactics. If she can't handle that, that's her problem - she'll probably give up or get bored on ragging you and your friends once you stop giving a shit. If she's really that bad, you need to talk to her and tell her to calm her shit or the friendship is over.
That girl seems to have issues and one day she'll end up having no friends with that kind of attitude.
Sera Myu Eternal Sailor Moon - Sera Myu Musicals - 95%
Mew Berry (Café version) - Tokyo Mew Mew Á la Mode
Usagi Tsukino (School Uniform) - Sailor Moon
Sailor Cosmos - Sailor Moon Sailor Stars
Bubbles - Powerpuff Girls Z