I didn't even know incest was a problem on Steam until hearing about this. I've seen some really perverted games on Steam in the past but something mild like incest being the rub is wild. Somebody on Steam's staff really has landlady fetish I guess.
It's not exactly Steam's fault, it's more of a "institutions around Steam" 's fault. Credit card holders can boycott platforms if they do not adhere to their likes and dislikes, for example. Governments could also get mad about it too, specially with Steam.
While I can understand rules for bestiality and lolicon/shotacon, cuz even if they're drawn animals and children they're
still essentially animals and children, and that could cause trouble for the platforms, much like SEGA fucked themselves in the 90's for allowing mature games being published on the Genesis, but incest being bundled together in the no-no zone makes no sense to me lol. And I'm not even an incest fan.
Patreon, Steam, Fanbox, they all add stupid rules like this because they don't want to get the short end of the stick. They're all business in the end of the day, so it's comprehensible. I think only a platform that's completely focused on mature content could maybe dodge that, and it's still just a "maybe".