It is his game and for whatever reason he decided to never do incest from start or decided to drop the incest for his reasons. Whether that was Patreon and risks he does not want to take for his reasons, changes in his own life that made him uncomfortable to add incest or found out when he tried to do it, it just did not work for him since not his thing. Any of those reasons are good enough in the end and up to him, since it is his game and he is putting in the time and work to make it.
Now do I think he should have teased it less if never intended to include it and hopes he tones it down in the rework if he never intends to include it, yes. I also belong to those it should be clear in game and not from words of dev outside of game. Would I be even happier if it was included at end, for sure, does that mean it has to be in: No. Just as long as we have no lesbian scenes or other scenes with males with those girls or even between the mothers either.
It is a game about the MC's adventures and if he will not have adventures with them, what they do in their sex life is uninteresting by definition. Let them have a relation in background, if really want to or feel sorry for them, but without scenes shown if really want that. Lets keep Mc the focus.