You fuck her.

You have decided since you got this far you should just go all the way. You tug on your dick a few times, then you pop it inside of her, it's really tight ( not that you expected different ) and she looks at you. Before she can say anything you have your hand over her mouth, all you hear now is muffled moans, she obviously likes it. Do you switch it up or do you continue fucking your friends sister?