Considering we still don't know what Phase 1&2 were for Becca's plan, with how smart she seems to be, pretty much all bets are off for what she knows. Along with how much and when she plans to illuminate to Frank.
All of this, of course, is taking for granted that Frank is indeed her dad.
If that is the case, I'm guessing she knows Frank is bio-dad, has for a long time, and it may have even fed her fetishes.
How she is/was going to see what Frank thought, and made him aware somehow, is still anyone's guess.
Doubtful it bothers her, or really ever has for that matter, but considering how he reacts to the idea of Abby joining the harem, she's likely cautious in how she wants to approach such a subject.
It is going to be interesting to see the justifications she comes up with after having repeatedly said, after the divorce, that none of them were actually related anymore.
In addition, she used that particular point in favor of her arguments for them to get together, so that might come back to bite her.