A couple has gotten bored with their sex life. The man cheats and later finds her wife cheating with the same woman which ends up in an FFM threesome with the couple and the same mistress. Novel comic by Seiren.
Gal na Imouto to Rusuban shite tara Kinshinsoukan ni DoHamari shita Hanashi | I was House Sitting With my Gal Little Sister, and We Ended Up Fucking Like Rabbits {Hennojin}