I love both the names Norah and Adeline. There's a bit of a stumbly glottal stop in Norah Adeline but it's not overwhelming. I think the reverse works too, and is cute as a sibling to Isla: Adeline Norah.
I don't hate the name Tobias, but it immediately makes me think of Arrested Development...especially with all of the buzz around the show this year. And while the character is hilarious, I'm not sure it's an association you want to make. I like Jules a lot.
Just my two cents. And Isla Harper is a gorgeous name!!
I do prefer the spelling Nora rather than Norah, but either way, I think it's a gorgeous name. :) I think that Tobias and Isla kind of clash, since they both have the long 'i' sound, but it doesn't exactly reduce it. I think either way, you can't go wrong. :)
Fun Story ~ I saw a girl named Isla, but it was spelled Iyeslah. And I always thought it was pronounced eye-lah, but they pronounced it eyes-lah. I'm not sure which it is, but the spelling was interesting, to say the least.
I like Norah Adeline, but the Nora spelling looks better with Isla in my opinion. I also adore the name Adeline, so I think either Nora or Adeline be beautiful first names. I don't like Norah/Nora in the middle spot though.
I agree with @heytherepigeon on Tobias and Isla clashing, but I think they're okay together. I love Jules, but don't I'd use it as the first name. Maybe as a substitute for Tobias, you could use Julian/Julius?