Well personally I wouldn't use them for sisters, since basically they're the same name. I used to hate Isabella/Isabelle/Isabel when it first started to become popular here in Australia, I just didn't see what all the fuss was about. Then I started to like both versions and couldn't decide which I liked better! I now like both
Isabella and Isabelle/Isabel, but I'd still choose
Elizabeth. I'd definitely use the nn
Lizzie and that's one of the drawcards for the name. The other drawcards being
Elizabeth Bennet from Pride and Prejudice, the fact that
Elizabeth is a Biblical name and also it's great meaning.