I don't remember but it's pretty far into the game. Almost at the end I would say.
The elaboration is in the spoiler above. The MC is the son. The father unfortunately doesn't know that the son is fucking his wife because of the way the scene is made.
It's also a very weird thing that we don't see more of this, all these harem games where the MC is supposed to be the alpha male and practically netori all the women away and still we don't have a consistent backlog of games where someone in the family willingly "lends" his wife to MC? As far as narrative go this could be easily done without changing the content of the game.
It would also be very interesting to see good old mom be the practice GF for the awkward son while you play as the dad trying his best to make his son into an absolute chad, can you imagine a management/vn game like that?