I would say that if I was able to work out that Trevor as Becca's father is quite unlikely, then Becca will have managed that too. It must have only been once Shannon and Trevor's wedding years dropped, Becca must have been within earshot and could then quite easily work out that something didn't add up if the wedding day was 2-3 years less than her own age. if Shannon didn't have the gestation period of an elephant cow, then it's pretty obvious.
And even though I'm not a fan of developers “distracting” themselves with side stories, I would argue that Lilly's and especially Becca's side stories are very relevant to the plot. The fact that stylistically we only learn things that Frank learns in the main part leaves a lot open.
For example, let's imagine that Becca tells Frank that she knows Trevor is not her father. Frank can perhaps find out an approximate duration, but not much more. A story that focuses on Becca and her trajectory can address that much better, so we can learn her modus operandi after such a revelation, more details on how that affected her relationship with Shannon and Trevor, how it affected her dynamic with Frank, and if so why it affected the dynamic in that way. Since we also get the thoughts of the protagonists, we have a better and unbiased impression of who Becca really is. This was also the case with Lilly's story where for the first time we really got to see how deep her feelings of inferiority towards her sisters are and also interactions in the past between her and Frank, which were briefly touched on from time to time but never a real flashback like we see here.