Now at least you said I
, cause indeed that is just a personal preference you share with a lot of people. To me, I do like some setup and showing some relation before the sex starts, but do not mind sex between them in the first episode as long as makes sense with the setup of the game as does here since you enter into a situation in the making for quite a while already in the game's universe.
Personally to me that so called incest taboo has become very non hot, since in no game or almost no game I have seen it made sense they break it in the end after having had so much trouble with it before, it just suddenly for no reason happens 20 updates later and still makes as little sense as if would have happened in episode 1 or even less sense since she and/or MC obviously have strong negative feelings about incest since successfully fought it for so long and no reason they suddenly fail
besides dev wants to close game and it is time to deliver the three scenes with Mom before game complete , but it is a much liked genre. (Not even talking about the glut of games that are abandoned just before that content would enter the game
).
It always feels kinda cheap to me and overly easy when it basically suddenly does happen after a stairway to heaven and unrealistic like hell. Here at least it is a situation where mom can not easily find another man safely or MC another woman without endangering them all. So faced with that situation with her son and let us not forget the manipulation of the sister, it at least makes some sense she has to choose between two in her mind bad options and losing her son and sending him away into danger and losing what he adds to their ability to survive as a group is an even worse choice to her while she actually also needs some male attention and sex as well to egg her on to the other choice.