When my mother was still alive, I always called her “Mама” or “Ma.”
I never called her by her first name.
As for games, I rarely play games involving incest (not my fetish), but when I do play them, being addressed by my mother by name just infuriates me.
Seems I'm in a pretty small minority here, but for me a major part of the kink *is* specifically the first-name basis. It signals that your relationship with her is now as an equal rather than her being an authority figure, which is an unconventional power structure that I find appealing, and which fits well with being in a sexual relationship with her.
I prefer still keep calling them "mom, sister, aunt, etc" since it's the point of the incest. Idk why but if in game I called them by name it feels like the incest just got less entertaining
Generally stick with "Mom" because that kink is probably the main point of the game/AVN. I have seen where the dev will use the whole "when we're out and about call me <name>, but in the bedroom I'm still 'daddy'..." effectively.