If it starts with cheating but ends up becoming a harem, then that is also okay for me.
Unfortunately the ones I've found aren't written to give off the cheating vibe, it's more along the lines of
You take the girl to the movies (finger her or get blowjob) then take her home and play with her mom while you all have supper. Then a few scenes later she is joining you and her mom. Kind of like in grandma's house.
But if I find any others will post them here.
Good luck though it is a fun niche.