Frank's sister has so far only appeared in scenes where Frank has told Abby about her mother. And even then only once physically.
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As for Shannon: I've seen Lilith's Longing, and even there you don't really see anything that would rehabilitate Shannon in any way. You only see that Lilith and Shannon have often clashed from an early age and she has also left mental traces not only on Abby but also on Lilli. There's a sequence in a dream scene that describes that well: Lilith is sitting in the classroom, Shannon takes the role of the teacher and asks her some questions. The questions are nonsense questions that don't make any sense and Lilith can't answer them accordingly. In the course of the dream, Abby and Becca "pop up" and can effortlessly answer the questions. I also mean that while Lilli is scolded, dream Abby and Becca are praised.
The scenario is absurd as such, of course. Both Abby and Becca have had their own conflicts with Shannon, and while the nature of the conflicts between Becca and Shannon are unknown, it is likely that Becca, as the more assertive of the three sisters, may have handled it better (I assume Frank's presence may have helped as well) than Abby, who until at least the last update (when she and Frank were at the fast food restaurant that is a direct competitor to Shannon's chain) was concerned about Shannon's reaction. With Lilli, on the other hand, it's the feeling that nothing she does and no matter how hard she tries nothing is good enough to get approval from her mother, even though, or perhaps because, she's actually a daddy's girl whose father ran off. I think that one reason Shannon was like that towards Lilli is probably because she and Abby got along so well (Frank can be heard saying to Abby in one scene that her mother would probably be proud that Lilli and Abby's friendship drives Shannon crazy).
So I think it might be helpful for all three of them (Frank, Abby, Lilli) to talk together about their trauma caused by Shannon. This would strengthen the bonds between the three and the nickname for Shannon (Shannunt) that Frank's subconscious has invented for her would definitely cause some laughs.
Other than that, I can only think of one thing that would work in Shannon's favor, and that's the fact that she tried to be a mother to Abby (before Abby started looking more and more like her mother). Given what she's done and how she's hurt others emotionally, I don't see any redemption for Shannon, at most if she were to deliberately take a bullet meant for Abby or Lilli, but I have no idea how the plot would escalate to that.