I mean, if you're fine with this it's up to you. Like I said, for me adding the father to this incest is a huge turn off and again, I would prefer much more having only the sister and mother. Incest is not necessarily everything with everyone, so having "all worlds". Since there are different "categories" in which there can be only the mother and son and so on, I'm one of those who if I need to add someone, I would only add another female relative and not a male. I couldn't care less about a male. Heck, I don't even like the gangbang kind of kink that surely others love, and in fact even about NTR I like to watch a single male stealing one single woman, not a bunch of them gangin' up on one.
And skipping is not even an option because if he puts half of the next video brother and sister and then sister and father, I can only watch half of it and, again, I'm really not interested in that.
Also, you're impling with "if he not only does the females but also his son, well that's just a complete story in my eyes." a yaoi relationship between father and son? That's... I'm sorry, but with this you're definitely the only one here thinking it. Who would even want to watch a yaoi scene?
(of course no offense to who's actually homosexual or anything, but as a straight person I can only turn around at that)
But as always, japanese are different and weird and they like these kind of things so me like everyone else will be the minority here. So there's no point in making a fuss about it in the end.