Lilli is definitely more the loud in your face person (as Frank can confirm not only metaphorically but also literally). Becca, on the other hand, even though they share certain traits as (half-)sisters (such as challenging Frank to bets they know they will win), reminds me of the Marvel and IDW Comics interpretation of the Decepticon Shockwave (for instance, his cold logic and calculating modus operandi is addressed), only in a human and feminine form.
That being said, I could very well see a portrayal of Misery starring Frank and Becca. Not necessarily as part of the plot but in terms of how far Becca is willing to go for her plans. Ironic enough considering Becca is the person who probably has the most writing talent.
Otherwise, what do you think the titles of Abby and Becca's side stories will be? Based on Lilith's Longing, we can assume [name] and a noun that describes an important aspect of the character. Possibly also in the form of a minor aliteration, although in Lilli's case that may have just been coincidence.
The longing in the title of Lilli's plot fit so far, since a big aspect of her was the longing for her father and a certain stability in her life. Her boyfriends were actually more gap fillers to fill the gap that her father left behind. She had two breakups in her storyline and even though she was upset at first with the second one, she quickly got over it after talking to Frank and accepting him as her father.
With Abby, I could imagine something like "Abby's Adoration", where her storyline would probably primarily revolve around her feelings for Lilli and her budding feelings towards Frank.
With Becca, I think something like "Rebecca's Revelation" would be appropriate, where we get deeper insights into her manipulations and mindset. Something that would probably be both fascinating and terrifying.
In such a story, I would also like to see the genesis of her plan that she has been working on for 5 years. She had fantasies with Frank before, as she herself admits, but I think that's when she started to see Frank as the one partner:
We know that Frank and Abby grow apart for a time. I think there were three factors at play: Puberty, Shannon finding Frank and Abby too close, and also Lilly with whom she developed a more than sisterly relationship. I think that was when Becca started her plan: Abby's age would fit the time, and we know that around the same time Becca started taking Abby's vacant place as "Frank's daughter" and spending more time with him. Frank himself says something along the lines that the two were becoming something like best friends at the time, and it wouldn't be the first time Frank failed to notice the interest of a female best friend.
By that time, the crush she had on Frank at I think about 15 or so may have developed into an attraction. She knew, of course, that it had no chance under the circumstances, but I think she would have realized that Shannon had developed a dislike for Frank and Abby. I wouldn't put it past her to try to track down Lilli's father and encourage her mother to have an affair by somehow contacting Trevor and getting him to contact Shannon.
If we assume that Lilli was also going to be part of this story, what were Becca's plans with Lilli during this time period, aside from what we know? Lilli was the only one of the sisters to have several boyfriends with whom she had experience, so what would have happened if she had gotten pregnant early, as Shannon did? Well I think that unlike Shannon, the role of the father (Frank), rendered by Becca and hammered into her sister's head so that wouldn't happen, which of course is not an absolute sure way. So I can also imagine that Becca was more or less directly responsible for some of Lilli's breakups (the only one we can be absolutely sure about is her penultimate breakup, since here Lilli was to blame, while on the last point we can't completely rule out that there Becca at least got the ball rolling).
To what extent Abby was part of the plan at that time is not clear. If she didn't know at the time that Abby and Frank are not father and daughter, she could have overheard a conversation by chance, for example, then that would be a bit extreme. At the same time, Becca has already hinted that she has at least a slight fetish in that direction. To what extent this was just a joke to lure Frank out of his reserve remains to be seen. However, she has never really, unlike this webcam thing, never really dismissed as a joke, so at least it can be assumed that there is a spark of truth to it.
In the matter of Abby, however, there was never really a risk, unlike with Lilly: Abby was, if I interpret the conversations in this direction correctly, always quite shy when it came to boys her age, even if she had a boyfriend. Lilli unknowingly took over the rest, since I trust her to have scared off potential boys who could take Abby away from her. Even if Abby wasn't part of the plan in the beginning, Lilli at least indirectly supported it with her "in your face" behavior towards people she didn't like.
But as I said, purely factually, Lilli was the only risk factor for Becca that something could go wrong, and Becca couldn't keep an eye on her sister 24/7, after all, even if there was probably some unwitting help from Abby there, too. Thinking about it, I suspect that the time between Lilli's first boyfriend to the present must have been a genuinely nerve-wracking time for Becca.
