Well and it is more likely that she is now developing those feelings and will jump his dick slowly without anything happened that explained why or makes it logical she changed her feelings in game after the start, unless she already had those feelings before they moved, in which case there is no reason she could not be on that journey from day 1 in the game or even already had a sexual relation before moving? Remember they are not strangers, neither Mom or Sis and living with him for years, not strangers that just met at start of game.
That is one of those weird arguments that it has to be slow or only start after start of game cause it is incest and then just showing your chest and doing some dishes or having a few more muscles and showing them off suddenly makes your mom ok with going on a stairway to heaven with you while she has/had a strong taboo against it at start game and basically nothing changed but now you are suddenly the one she sees as the only option for her for a sexual and/or emotional relation while looking like a supermodel. That is porn logic, not real logic and completely unbelievable if want to be/make it realistic .
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Now if she had hot hormones that she could not control either in years before and had been bringing different males home all those years before and then would be dropped on an uninhabited island with just MC, that might make it believable she would break that taboo if has it strongly over time and having nobody else around to scratch that itch.
Now if you like that porn trope, fine we are all allowed to like what we like even if you love Marcus having the action, but the argument it is more realistic or makes the story more realistic is laughable.