Can you imagine the context where 3 randoms who are the daughters of an ONS that you had 20 years ago go to live in your house just because? I would tell the girls to fuck off lol.
Most "Landlady/Roommate" storylines don't make sense. Because they're not written to make sense. They're written so that everyone who's playing knows exactly what the secret code is, but the dev still has just enough plausible deniability to get away with it when the Patreon goons come knocking on the door asking if their game has any
prohibited themes.
It's the same reason why ALL incest patches are "fanmade" - because Patreon has absolutely banned devs for releasing official incest patches on other sites, even if the base game doesn't have incest. Because in their eyes that's endorsing/enabling the content. So devs who want to make an incest game and still make money via Patreon have to pretend
Having three girls who are completely unrelated to you and the daughters of someone you had a ONS with 20 years ago move in and become your "wards" or "houseguests" is the same Landlady/Roommate fiction, where you're supposed to ignore the unreality because we all know what it really means. And then "someone" can release a "fan-made" patch later to fix it.
Incidentally, that's why people like us should never openly admit if a patch is made by the dev or some rando. Because some devs have absolutely gotten hit for that sort of thing in the past. The Patreon goblins definitely check sites like this one to see what they can find.