Only thing I can think of is the dev's own moral compass. To some even when imaginary stepcest is fine but actual fucking blood relatives is cringe-inducing, esp when you are the one writing the story, posing the renders, etc. Probably for the same reasons incest fans find incest hot but stepcest annoying: with stepcest there's no additional associated risks like inbreeding, getting outed and the social stigma that goes with that, going to jail, the forbidden fruit factor no longer applying, etc. You're basically just fucking a friend with the only difference being they just happen to live in the same house as you.