DJSG-020 - Caught by my sister while masturbating with her panties

I got caught peeking at my sister climbing the stairs. She was furious, and it turned out she even knew I was masturbating with her underwear. As I hung my head in shame, she asked, 'Is this what you wanted to see?' and showed me her panties. When she noticed I was fully erect, she said, 'Look how big you've gotten,' pulled down my pants, and started gently stroking my throbbing member while staring straight into my eyes.