It's a little frilly but not too much. I think it is sophisticated, although I prefer Annabel. The Annabels I have known have all been the opposite of frilly in real life!
It does go with your other children's names. I don't care for Anne or Nan as a short form. Nan wouldn't work here anyway as it's one of the most popular names for a grandmother (more popular than Grandma in the UK I'd say).
As for a twin sister:
Annabelle and Florence, Annabelle and Iris, Annabelle and Georgina, Annabelle and Felicity, Annabelle and Josephine.