We have a DM who's very hooked up on Japan on various aspects. He also downloads ero-gemu on a daily basis.
Just as a FYI, on Japan incest is damn common.
I recall on one campaign there was this one NPC we picked up along the whole thing that desperately sought a certain woman. Turns out said woman was his sister.
One campaign later, same campaign world, she had children. HIS children.
So, yeah. Not as common, but there you go.
Also, boning your mom, while still technically incest, is not something Japan frowns that heavily on. (Heck, I think over half the doujin out on the internet have incest in them, and out of those doujin, at least 70-something percent has young boys boning their moms...)