Incest has never been and likely will never be a theme focused on in this game. If you're really absolutely dead set on incest you'd probably have to mod it in yourself. I suppose you could maybe mod in some bullshit revelation that Mystique gave you up at birth similar to Night crawler thus she's your mom and Rogue is your (adoptive/foster) sister.
But really that would be a convoluted stretch for the sake of shoehorning in a niche fetish that's already more than well represented enough in other games. Oni already takes ages pushing out releases so it would be a terrible idea for him to spend extra dev time on making a contrived incest patch that would only serve to jeopardize his income source if Patreon ever managed to figure out he made it himself.