I usually prefer bro/sis, but it depends on the circumstances/ages. F/D stuff often just feels too groomy to me. There are some ive seen that did it well (not super believable, but meh) where the daughter initiated it, so it felt less groomy.
And yes, this is sexist, but I feel M/S is less bad. Although I hear differently these days, any 9 year old boy I knew growing up would be asking for high fives if he lost his V card to a most-not-ugly girl. Im not saying boys cant be raped, but I would bet a finger that 80% of those high school boys who were ploughing those decently cute teachers that made the news and pretended to be all sad and hurt about it were 100% faking so their mom didnt bust their ass. Male teacher touching female student? Cuffs. Not-fugly female teacher letting the male students pound it? Hero. She made some dreams come true.
Long story short, 99.999999999% of these M/S incest the mom is "hot" (even if I dont think so since I am not into this massive fat-bag milf stuff), and the buys are always into it so I don't get the same creepy feeling I do from some/most F/D cest games. Although I have seen some bro/sis ones that felt groomier than the F/D ones. Even DMD before it cuckatized itself... She was supposed to be "of age" in "his country". Which If I remember, she was 18 but her country required 19 to be an adult? But she acted like she was 8. "Sex? Oh, thats a peepee?" That one felt a bit groomy too and I just kept telling myself "Legal age. And tits bigger than basketballs. Not his fault she has the mental capacity of a preteen. Ok, so it is, since she has half his DNA, but... fuck it, let him hit it."
I have seen a few where it is older sister that were surprisingly hot, but I think that was because it relit the old preteen-young-teen fantasy of a slightly older girl. Wanna give the younger brother a thumbs up because big sis is making sure he is taken care of
Oh, forgot to note that in the rare few where the daughter initiates it, then the F/D ones are sometimes pretty good. But the problem with these is, to do it well, the daughter has to be less innocent, which ruins part of it for me. So its conflicting. If the girl is younger, I need her to be not-innocent and instigating shit to get rid of that icky feeling.