Your sister lied to her boyfriend that she is pregnant to make him propose to her and he did. But now she has a huge problem, because everyone will expect her to be pregnant at the wedding and she is not! You have to help her! You look like her boyfriend anyway, you are her favorite type! You have to start fucking her and filling her up with your cum to get her pregnant as fast as possible! This way no one will suspect anything! You are family after all and you will help her with your hard cock, right?