Why can't it be Liam who comes home and almost catches Sophia and Sam? That would be logical from a narrative point of view and could play a role in Sophia's mind in the subsequent Cathy event.
Liam may eventually find out, but it probably won't be until the novel's end if L&P ends getting that far. On the other hand, Dylan is slotted for Cuckold/NTR, as you can find it in the point system and a couple of choices throughout the novel. Some of these choices are The pool scene (swimming lesson), Trampoline, Mall event, etc.
I would like no more than Liam to be the primary source Cuckold/NTR path, but that's not how L&P wrote the story as the points show, and just like the swim lesson, It would make sense that Dylan catch them from a distance for points.