Peter takes over

You see your sister's body shake.

She grabs you and kisses you full on the lips. She mashes her tits against you and grinds her hips against your dick.

She pulls away and walks over to your mother who is aghast at your sister's actions.

Your sister then kisses your mother, whose submissive lesbian's tendencies take over, and kisses back.

Your sister rips open your mother's dress and sucks a nipple.

Your dick gets hard.

Your sister sees it and smiles.