I read [Izayoi Seishin] Boshi Double Soukan again and remembered a manga that i think was an incest one. With mother or stepmother who was an ikebana/flower arrangement teacher. The scene i remember was one, where they had sex in middle of a class, where the women went out for a break and her son/stepson has sex with her. The students wondering where she is. I think this wasn't a one shot, at least 2-3 chapters long.