I think
Elizabeth and
Isabella are way too close for a sibset.
Isabella is a form of
Elizabeth. It may not bother most people, but to me it would be the same as having a
Jennifer and
Guinevere in a sibset. As for which one I prefer, I have always loved
Isabella, but the popularity of it has turned me off.
Elizabeth is my middle name and I am only now learning to appriciate it. I would pick
Elizabeth with an uncommon nn or an unusual middle name(or both!).