So I am planning on writing 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 or maiden. this charactor is submissive, giving, caring, etc. if you use this, you 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 you think is best.

can't wait to read your story

Find a list of baby names used in 1985, or whatever year Bella was born in, if you want to be accurate.
If you don't care about that, pick whatever you want.

Nina ??

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