I'd also like to note that this will make for some interesting situations in the plot:
Becca as Frank's daughter makes sure that the sisters are in even more of a patchwork situation:
Not only does it make Becca Lilly's half-sister, which was already pretty obvious, Trevor as Becca's father has always been a very short-term time slot, but now Abby's adopted sister, formerly adopted stepsister (is that a thing?). By both becoming pretty much Frank's daughters, one biologically, the other by birth certificate, their relationship became a bit closer. I can imagine Lilly going into a negative spiral as a result, though. I also think it will give Shannon new material as an antagonist.