My wife has a younger sister who is 18 and is a senior in high school. We went over to stay with her family for Christmas and were staying in their spare room down the hall from my sister-in-law (I'll call her Sarah, not her real name). Sarah is a cheerleader, skinny, perky B chest, great body. Not quite as hot as her older sister, my wife, but still hot.
One night after we had all gone to bed I got up to go to the bathroom, which is in the hallway and connects to Sarah's room. Before I could flip the light switch on I could see the door that connects to Sarah's room was cracked open slightly. I couldn't help but peek inside.
My heart nearly stopped beating when I saw she was laying on her bed, completely naked, masturbating. She was facing mostly sideways to me so I couldn't see her pussy, but her hand was down there rubbing and her other was pinching and rubbing her nipples. There was barely enough light in her room for me to see from her night light.
I stood as still as I could and watched, but my penis was instantly hard and begging to be touched. I carefully slipped it out of the fly of my pyjama pants and started stroking. I could have cum in seconds but held off, watching her.
After a minute or so I was able to hear her breathing heavier and making quiet moaning noises and then she pulled her knees up higher and I could barely see her pussy and then she shoved her hand down and began shoving a couple of fingers in and out and I could hear her wetness and I almost exploded right then, but held off by stopping stroking.
She fingered herself quickly and her moaning and breathing got a little louder. Then she gasped and started making an 'Uh, uh, uh, uh, uh' noise and her upper body lifted off the bed, her body bending at the waist as her whole body shook while she started cumming. (She looked exactly like my wife when she cums.)
I gave my penis one or two strokes, cupped my free hand under the head and immediately began shooting a load of hot cum into my hand while she continued to twitch and grunt quietly on her bed, then collapse back onto her pillow and relax, hand still slowly rubbing around her pussy. I flipped the light on and shut both doors and washed my hands before heading back to bed.
After I got into bed I was still hard again and my wife noticed and said she was horny, too, so we quietly made each other cum in a more advanced fashion that can't be mentioned here, but as I unloaded my second cum in the last 30 minutes deep inside my wife I was picturing her younger sister writhing in orgasm on her bed.