Depending on MC age in the story, at my current age I couldnt get a hard-on for an 80+ year old Mom so would choose Sister. (Prefer in that scenario my Sisters Daughter)
IF MC is a 18-25 yr old and the Mom is Hot then Both. Prefer Sister in Most cases in the games I have played, its nice to know we can be together for a long time and I'll still be attracted to her.
Having a 40+ year old Mom has a limited shelf life on continued Intamacy. Mom from Complete - Ren'Py - DAZ - Lust Theory [v1.0.2] [Inceton Games] was amazing but still wanted my elder sister Jessica the most, she was beyond fuckable. Wanted to breed that girl forever.
Between just these 2, mother. The older I get, the more into milfs I become.
Overall I like, father/daughter stuff best. But I'm too picky and there isn't a lot of it I really like. I like it to be wholesome, and I don't find that nearly as much.