Per the theory-crafting on the previous page, re: Becca's role, I think it boils down to one question: Is she malicious? A person can be manipulative and calculating and not be malicious; they could be the planner and organizer in a relationship or family unit. The whole 'quiet and shy' aspect that other people see of her simply could be that she is focused inward, family-wise. She has her circle of affection and emotional nourishment and takes care to maintain it.
I don't think she shoved Lilith and Abigail together so much as encouraged a loving bond. And I don't think L & B are soul mates or any such thing. I think they're step-sisters with benefits. Lilith has maintained other relationships, as well as a sexual history; she doesn't seem locked in. And Abigail never seems possessive and seems to have interests of her own.
To hazard a guess on an entirely fictional character, I'd guess that she grew up with a non-nurturing household, has the sense of responsibility that many oldest offspring do, is aware of her mother's selfishness and amorality, and while not particularly judgmental about it, will not participate in it or give it a claim on her life. Still, she's shaped by it to some degree. I believe she wholeheartedly loves Lilith, but will callously push her. And maybe some of that is appropriate and needed. Lilith IS kinda a mess. And let's face it - sibling rivalry is perhaps inescapable.
And regarding the MC, I think she's firmly decided on him, and as was suggested, willing to manipulate things to keep it together. And all three girls have exhibited some degree of sexual fascination with MC. I think this is what the Lilith bet was; to nudge her in that direction and gauge her reaction. It was a bet with the MC, yes - but also a test for Lilith.
In short, I do NOT think she's malicious; quite the contrary. But she IS decisive and was raised with a certain sense of amorality. The MC provided some moral compass (and unconditional love), and she responded to that. Plus, he's apparently hung like a bear. So for an extremely smart, calculating loner-type, it may seem quite natural to try to shape the Affection Unit to which she's committed herself. I suspect she's an "All Fair in Love & War" type.
Anyway, thanks to those in the earlier discussion. Just thought I'd add my own thoughts. This is a superb story; for the genre, and perhaps beyond.