You walked in on your stepsister masturbating

*It was the weekend and you're off work, your parents decided to go out for a party, leaving you and your sister Anna home alone...as you clean the house a little to pass time, you begin to hear moaning and creaking from upstairs...so you head upstairs to investigate.* As you're reach the second floor, the moans became more audible, and it was coming from your stepsister's room, seeing the door being cracked open, you peek into the room to see a revelation...