Its 50/50 on that one. If you look at how old Becca is and work backwards to when she must have been conceived, Frank was sleeping with Shannon. Is there a chance she was cheating? Yes. Is there a chance he could be Becca's father? Also yes. It is also entirely possible, given how much planning Becca has done, that she crunched the numbers and figured out who her father was. She has said she has finished other steps in her plan that she can't tell him about yet, that could be one of them. So, yeah, he could be her real dad, he might not be her real dad. We just have to wait and see.