You saw her.

While you was masturbating, you looked over and saw your sister peaking though the door at you. She looked pretty flustered before she left. Deciding not to finish and you wanted to see what she was doing here you decided to put your pants back on and walk to her room.

When you got to her room you placed your ear up to the door and heard her moaning. She must have gotten turned on seeing your cock and decided to take care of it. Your not sure why but you dropped your pants to the ground, grabbed your cock and started to play with it to her moans. Her sounds was really turning you on and your cock was extremely hard.

You did not think you might last long. While masturbating you feel the door creaking open. "Oh shit it must have not been closed fully" you think as you fall into the room with your cock hanging out.