The most reasonable step that a dev would have to take is to make Mary is the next wife of father after the death of the mother of MC, and Tiffany is his half-sister. But as we all know very well, our wonderful author (author's) is not looking for simple ways. The fact is that such a scheme of relations: MC - stepmother - stepsister has already been perfectly implemented in the game "Summertime Saga", which has long become mainstream for games of this kind. After the protagonist's friend, fat Tash (his analog is Eric in "SS"), was miraculously teleported to "TT", the plagiarism became visible to the naked eye.
Now, attention the question: When MC gets drunk in the trash and fucks Mary from behind in the washing machine, we, as players of this beautiful game, should consider this event a corruption or rape?