Unless i am mistaken, the norm in the incest genre is where mom is fondled and attacked by sonny while cleaning and doing the dishes. Eventually, she gives in to passion and responds. I just downloaded a Maki Tomoda title- URE-056, where to my surprise she initiates the action by rubbing up her son's leg under the table, and later on taking off her panties and inviting him to explore. There must be more like this in this genre- any you can think of off the top of your heads? Thanks. 
