To me if there is only vaginas involved I don't give a shit if it's incest or not. I's all the same.From what I saw on the numerous 'lol not incest patreon' games that switched from relatives to step-relatives, the overwhelming reaction to the change from incest fans is that step is not incest. So don't bother with trying to sell Yen/Ciri as 'step-incest'. It's not incest and they won't care.
As for lesbian and straight incest, dunno. 99.9% incest games are son/mother, brother/sister or father/daughter, meaning they have a male protagonist. I know only of one game with a female protagonist and and potential F/F incest, it's 'A wife and mother', and there are like three people cheering for mother/daughter incest in the thread. It seems to be a very niche fetish.
About Ciri-Yen being daughter-mom, I only played about two chapters of wither 3 and read no book but how much time did they even spent under the same roof? To call them having sex incest sounds far-fetched.