I think in the long run there are only 2 ways we can find out the truth:...
Hmmmm.....maybe......I think option 1 would work with the story (that we have so far) better than option 2, but not by much.
I'd have to check, which I am too lazy to do right now, but I think I remember both of Jess's parents being on-board with kicking her out. If that is the case, I doubt either would hold onto her personal belongings for so long.
Maybe one/both (are both of her parents still alive? I forget) found some stuff in the attic after Jess passed, and hung onto them then out of regret though. I personally believe the story is heading in that direction with them giving Abby's bio contact info for Frank.
I've seen way too many people suddenly want to "make things right" when they get closer to the end of their lives, and they have to be 60+ (based on Frank/Shannon being 39, and Jess being in the same grade as them).
I can see Jess's parents suddenly wanting to be in Abby's life again, especially since that would be all they have left of Jess. They could easily be using the bio like a testing ground to see how Frank/Abby are going to react. That is really the only logical reason for the bio to ever have been in contact with Jess's parents in the first place.
As for Shannon getting pissed? I dunno' there are so many opportunities for that to already have happened, and she still hasn't mentioned it. I can see it happening; Abby looks/acts so much like Jess, that Shannon might actually forget who she is yelling at, but I'm not sold on the idea yet.
My guess for the future....
I can see Shannunt & Party-Girl teaming up, or at least see Shann trying to, but I don't know how well that would work out. We would need to meet her/get a look at her personality, but I doubt that Party-Girl would keep Shan around long enough to get manipulated by her. She might be willing to use Shannon, but....hmmm.....we'll have to meet her before I have a real opinion on her.
My personal take is that this is going to end up being a four-way, and (sadly) not the sexy kind involving Frank and the main girls.
1) Shannon isn't finished yet, and is going to go absolutely ballistic very shortly.
2) Abby's bio & her grandparents have their own agenda. I think it is going to be the regret route, at least for the grand(s), and until it comes out that Frank & Abby are a thing (if/when that happens).
3) Party-Girl is going to want more money, she doesn't really care about the house, and the lawyer/brothers are going to be involved in that arc. It might even end up with the house being sold back to the oldest brother, and Frank gets to "rent" it as needed/wanted.
4) Somewhere in there Lily will have to face the Frank/Abby situation. Abby might be able to calm her down eventually, and will probably be the one that sells her on the idea, but Lily is going to be just as ballistic as Shannon is about to be when it happens. That might even be how Shannon ends up finding out....