Kek, your salty, but I bet I can come up with a straight up bad ending that's not at all positive BUT gives closure in a satisfying manner.
It can be fappable too although, honestly, that depends on whether or not you fap off to birthing. Speaking of: Death due to childbirth. Maybe it doesn't have anything to do, strictly speaking, with the fact that they are having an incestual relationship since the baby could have been the father's and it could have still occurred, but the simple fact is....the baby belongs to the son and, if they hadn't started things, it would not have been born.
But that's not their fault, and anyway the mother could never hate any of her children. Even the one that ends up killing her. But the son, you see, doesn't see this straight up...nor can he bring himself to hate his sibling/child. As it's the last thing he remembers from his dead mother, it makes the more tragic since he can't legally claim it since incest is looked down upon, making the him go to the "suspected" sire. Yeah, the person he personally blames for the divorce and hates; His own father.
But he figures that he would play along simply so that he'll have a chance to raise his sister/daughter or son/brother.