Thank you all for your reply!! They were all very helpful, and we were able to narrow down to a few. 
So we definitely like Isabelle better, like a lot of you. (And yes, I totally agree that Isabelle would be a better choice to go with Jacob because I’m personally not a fan of the Twilight series! :p) Here are the middle name candidates:
- Isabelle Florence - very VERY pretty, but I’m not sure about the middle initial “F”… Am I weird, or does anyone else think F looks a bit awkward when you write the first name - middle initial - last name combo on paper? I love it when I spell it out, but Isabelle F. seems a bit odd to me…
- Isabelle Lorraine
- Isabelle Christine
- Isabelle Celine - Too brand-name-y? I’m not too into names like Chanel (it is pretty but not my thing).
- Isabelle Felicity
- Isabelle Rosamund - I love the name Rosamund, but I’m not too sure because most webpages seem to say that the meaning of the name is “horse protection” while it actually means “rosy lips” in German and “rose of the world” in Latin… I do love Isabelle Rose, but Rose as a middle name seems too common these days.
- Isabelle Fleur - Sweet and pretty, but I wonder if people will have problems pronouncing her name correctly? We don’t have any issue within our family, so no problem there, but like with Florence I mentioned above, the F middle initial seems a bit off-putting to me.
- Isabelle Olivia
Which ones do you like best from this list, or would you suggest something else? 
Thank you for bearing with my indecisiveness haha!