There were points in the walkthrough that says that S will be more accommodating of other men if you make choices that she doesn't like. So we can expect that she might've been impregnated by the guy your mom matches her with or by Vicky's boyfriend since he's been eyeing S from the start. Kinda unshown NTR.Same here, she also suddenly has a belly, references to been hormonal (backed by crazy mood swings and demands), also when talking about Anal at pool with Vicky and Sis one of options was to do Anal near giving birth. We got NTR'ed man and we going to be raising someone elses kid lol and not even the joy of a single creampie.
Other scenario: you creampie her repeatedly and she got pregnant but still not your kid. How's that for screwed over NTR? Every character in this game are assholes except for S who's been dragged around through all the squabbles. So many possibilities.