I don't know if this was something said in game/in side-story or just speculation in thread, but I remember something said about how much more bitter Shannon became the more Abby looked like Jessica.
It is mentioned at the beginning of the biographies. In this case, I think Shannon's bio says, "Shannon tried to be a good mother to Abby for a time, which changed when Abby's appearance began to resemble her mother more and more.
On Shannon and Frank: I think he massively underestimated the extent of the dislike between the two of them, some might categorize that under "naivety is a fool's blessing", but actually some of it might also stem from the fact that neither of them ever said what the problem was between them, implying that it was something that could be a factor of massive shame for both of them. But yes, Shannon was a bully and I still assumed that part of the reason she dated Frank back then was because she knew she could hurt Jess with it. I think the only positive thing we can say about Shannon is that she tried to be a mom to Abby for a while, a phrase that comes from her bio and is therefore a neutral source.
In that context, I always try to compare it to "What if Jess was alive and Shannon's daughters were living with her and Frank?". I think that Jess wouldn't be entirely happy at first - understandably - but once she got to know Becca and Lilly a bit and saw Abby befriending them, that she would open up to them and quite possibly be a better mother to them than Shannon.
I would even say that ironically, since she has the most resemblance to her teenage mother, that she would probably get along best with Lilly since they have certain similarities.
Lilly was abandoned by her father -> Jess was disowned by her parents.
Both are very unlucky with men (no, I don't mean you, Frank)
That said, I'm convinced Jess would find the irony of her worst enemy's daughters befriending her own daughter, and the idea that it would have driven Shannon insane, highly amusing.
So, please please please, CNG, when you're finished with Single Again, do a side story that addresses such a potential scenario.