You and your step-sister always play these gross games where you have to jerk and you're not supposed to cum to prove that you don't like it. She calls you into her room and she's wearing a see through dress. You're instantly hard and it's time to hold your dick in your hand and try not to stroke. She only lets you stroke when her ass is shaking. As you're getting close, you hear the garage open. Your step-mom is home and your sis doesn't have a lock on her door. You try to put your dick away but your step-sister whispers angrily to keep stroking and sniff her ass. You end up cumming all over your step-sisters ass with your step-mom right outside the door.