Why the fuck is incest such a big deal here anyway? Might be in the minority who likes the fact the MC isn't trying to fuck his family in every game
it puts all main characters in the same house, gives automatic strong tension between them, has milf(s), and is relatable as everyone has lived in a family. different age women for different tastes. it basically writes itself, yet you can extend the cast outside the house easily. aunts, neighbours, work friends etc... it's an even easier setup than the usual high school setup.
if you want the same character structure without incest, you first need a plausible story to get them in the same house. even with step-family you already have problems getting the family formed plausibly. then you need to create backstory to stop them from fucking each other, so you get tension. tension amplifies the release. and you probably want them to have a personal connection which is gonna take time too.
it's just so easy to start with a family.