I'd more than
it if Bob would've already fucked his girls even before the start of the action in the game and MC slowly starts finding out with definitive confirmation late into the game.
Bob could've been the one who teaches them anal, to only use their daddy-hole and avoid pregnancies. The small one could blow him randomly just because she LOVES the taste of her daddy's cum while the big one could just be an anal whore for her daddy's THICK cock. This kind of NTR would be fucking awesome and the dynamic of MC slowly finding it all out and later helping his dad finally cum inside his girls pussies would be legendary.
I would not like it if rando strangers get to fuck the girls.
Nothing like that will happen, though. The action needs to be kept vanilla because most of the audience is very snowflakey and self-inserts.