Peeked on Mom Masturbating

When I was playing army in the house, 'hiding' from the enemy, I found one of those old-fashioned hand massagers in my parents' room. You know the type; it has a motor and those springy grips that go over your hand. It was designed for back massage. It never occurred to me then that my parents used it for sex.
The summer I turned 13, my brother and I were playing outside, throwing the frisbee. The disc landed by the side of the house, and when I went to retrieve it, I heard something through my parents' bedroom window. The curtains were drawn, but I could peek through a small crack into their room.
I looked in, and saw my mom standing by the bed, her pants down below her knees, and she was moving her hands up and down between her legs. She turned a bit, and I could see she had the massager on her hand, and she was rubbing her fingers with it over her pussy.
I motioned my brother over, and we watched as she stood and worked the massager on herself. Her butt cheeks would clench and relax, and finally she started rocking and hunched over, grunting and moaning. Her butt cheeks clenched and her knees almost buckled as she had her orgasm.
My brother and I were astonished. When you're a kid you don't think your parents enjoy sex, much less masturbation. It was a young man's thing, right?
We quietly but quickly got back into the yard, throwing the frisbee. Mom came out a few minutes later with some lemonade. Needless to say, that night my brother and I were furiously masturbating (separately), and without saying a word, we knew what got us so worked up. After that discovery, my brother and I would find times to (again, separately) sneak into their room and make ourselves cum with the massager. Sometimes our parents were in the living room watching TV. I would stand guard while my brother got off, and then he would do the same for me. Very, very intense orgasms.