OK.

A lot depends on the quality of the shoot, attractiveness of the models, etc., and whether it's US models, but assuming that they are US models, I would guess the range that content buyers would pay under those circumstances would be between $1000-2000, perhaps a bit more if it's a really stunning scene.

Also, keep in mind that if the models are real twins and they are having sex (as opposed to just j/o next, a lot of studios/sites won't touch it because it would be incest and considered obscene in many places.