So I am planning on Письмо a twilight fanfiction and...

well, different people have different ways of picking out names for charactors. mine is naming the charactor according to their personality. Like in one of the stories i'm writing, i named my lead charactor cora, which is greek for maid или maiden. this charactor is submissive, giving, caring, etc. if Ты use this, Ты should basically pick the name that fits bella's twin's personality (luv the idea of a twin, btw.) if not, than just pick the name that Ты think is best.

can't wait to read your story

Find a Список of baby names used in 1985, или whatever год Bella was born in, if Ты want to be accurate.
If Ты don't care about that, pick whatever Ты want.

Nina ??

i Любовь the name sara but only use this name if bella's twin is kinda girly или if Ты want a long name i would suggest maybe Coraline?? but then again she would have to be kinda girly