I totally agree, it's hot. And if these things would appear in more VNs I'll say nothing against it till it's not ruining story. Maybe not mother-son, but to each his own. My words were pointed at that:
That's the reason for my comment.
I see you remember what you taught in school, that's great. Let me tell you some other school things. Everyone knows what selection is. That practical discipline gave us all the breeds of cats, dogs, cuttle, even wheat, tomato and potato. One of methods of selection is inbreeding (not the only one but one of main methods, along with pure lines method). Need I remind you what that word means? That method provides us with better food and our favorite pets. Increased chance of illness? Well, yes, more of that, some breeds have congenital weaknesses and/or deseases (like cats with mono white fur usually have troubles with hearing, it's genetic). It's consequence of serial inbreeding through many generations. The same true for human, serial inbreeding through 5 or more generations brings us... not healthy descendants, to say it mildly. But at the same time babies of two healthy male and female, brother and sister (twins - even better) will have heterosis, i.e. that babies will be even healthier than their parents.
Summarize it all, biologically speaking, incest is good, but only if we're speaking about few generations, then it's undoubtly bad. It's not societal thing.
Another thing, clearly not school material. Old testament, destruction of Sodom and Gamorra. Rightful man Lot and his daughters lost their wife/mother. The next thing they do - daughters sleep with their father and each one give birth to incest baby. It's in Bible, classic. And it's societal thing.