According to what I could read that Jill keeps the photo when you choose Isabella
There could be a revalidation I think Jill is not going to want to lose but this would make Jill have a 180° turn in her personality but if you think about it Jill needs to be more aggressive with what she wants
If you're looking for another poly I didn't think it's happening anytime soon in the game's story
Communication is needed first and DPC loves the drama and other than we have to see if James is still alive or not for now it's too early to draw any conclusions
For example there is 1 factor very important that Josy/Maya/Sage share they like to see the MC fuck another girl
So, look at it from Jill's POV.
She doesn't know what a "traditional" relationship is like, and as we saw, she went pretty much in a straight b-line from kissing to sex once she realized what her heart wanted.
We KNOW she loves Bella, maybe not that way, yet, maybe she doesn't know that the does. Unless i missed a reveal, we do get a hint that SOMETHING happened between them in bed at some point.
From Bella's perspective i think it will be even simpler, we already know she is a freak, and loves Jill, and has strong emotions for MC.
I see both of these as sufficiently established, while still requiring more development, ofc.