I have no doubt this subject has been brought up before, but I personally haven't seen it, so I don't know.
For me, the thrill of the incest fetish is that it is not just a taboo, but a disturbing and disquietingly tempting fantasy. A fantasy wherein there is a forbidden yearning, a shameful spying, maybe a daring phrase. Perhaps, in an impossible and grotesque moment, an accepted kiss or touch. The relative in question is absolutely off-limits, of course, but are they really..? And why would you? How could you? And yet...
Let's say, for example, a boy knowingly has an incestual relationship with his consenting biological mother. We percieve this as taboo because she's a direct and very close part of his dear family. So when she becomes just another notch on his bedpost, the horrible forbiddenness of it drains away to just another conquest.
I understand that my point is a mismatch for this game, since it's a dating similator that more-or-less gives the MC a harem. My point is that I'd like to see an incest game where the mother-son relationship continues even after incest has breached, and even when the forbidden couple are alone together. They retain their mother-son social hierarchy. I don't think Summertime Saga fails at this, but I believe it pales in comparison to Dating My Daughter. I'm not dissatisfied, but I get a little disheartened seeing mum lying on the bed, seeming to exist primarily for the gargantuan dong of her progeny. Still, a first-rate game.
Thanks for taking the time to read and consider this.