http://www.doujintoshokan.com/read/Abnormal-Family/shinkage/0/1
Now, this is Incest!
Holy fuck, I like the plot so much. A little boy, named "Maa-kun" just came home. His mother is counting their family's in and outcome of the month. Suddenly he grabs and plays with his mom's boobs, after a while of so-called "game", they actually fucked each other... And his father came home and fucked his wife with his son. Pretty weird, but very good.