If this is the case that's kinda going against the whole message of the story. I personally wouldn't like that. And I have nothing against incest in AVN. I just don't feel that it would fit into this story well.
A message of 'family is what you make of it' doesn't necessarily preclude blood family (and Rebecca and Lillith are sisters or half-sisters). There is also the narrative pattern of 'characters perception of their family undergoes radical realignment' that would be continued.
Also, it's all incest, regardless of the definition Patreon uses. Laws and cultural taboos about incest are not limited to blood, and considering the real life prevalence of cousin marriage arguably are not really about blood at all.
Alternate possibility; Rebecca has mentioned a plan five years in the making, which would put the start of her plan right around her 18th birthday. There is the possibility that she did the math a long time ago, and got a paternity test when she could do so without Shannons permission/knowledge (Ie on her 18th birthday)*. Narratively, either a positive or negative result could be written as her inspiration
As a weak aside, the flashback campfire scene does kind of show Franks sister dip into manipulator mode. This could end up being a 'hint' that Rebecca gets it from her aunt Crystal. It could also very well be that having Frank as the PoV character makes anybody with the social intelligence of a salted slug look like a Machiavellian genius in comparison.
*I have no idea if that actually matters legally speaking, nor does it really matter for it to be a justification in the story.