I always liked the name Isabelle, it was even on my "LIST" before I started birthin' babies, but now I don't think I'd ever use it because it's getting too popular, especially in the last three years. I don't know if it's because of Twilight or what, but it's definitely all over.
That said, I wouldn't advise you to not use it. I'm INSANE about other people using "my" names. My favorite name used to be Violet, but now because of the Afflecks I could never use it. I have panic attacks thinking that someone is going to steal my new favorite girl name before I get to use it, even though it's extremely uncommon and a little weird. I've only heard of one person IRL with the name, an old lady with huge hair who karaoke-sang "Stand By Your Man" on my honeymoon cruise (but we liked it before then!). I even kinda regret naming DS1 Eli, because I'm hearing more and more kids with that name whereas before it was only Elijahs and you never heard Eli. Pretty sure I'm not going to have to worry about name popularity exploding with DS2 Wolfgang!
Anyway, Isabella is the one that's really getting super popular. You don't hear Isabelle as much, even though it's very similar. If I were you, to make sure Isabelle wasn't misread as Isabella, I would spell it Isabel or Isabell maybe even. I really dislike "unique" spelling, but I think those are both acceptable variations.
Your DDs all have very pretty names!