Caught my sister multiple times

This happened many years ago.

My younger sister stayed at my place for 2 weeks, but because my appartment was so small we had to share a room. It was no problem for either of us. She got the bed and I slept on an old mattress on the floor.
That night, about 20 minutes or so after we'd gone to bed, I started hearing soft sounds coming from her side of the room. She was breathing heavily and every now and again she'd moan very quietly, clearly trying to suppress the noise.
It was clear to me that she was masturbating. She must have thought I was already asleep and I didn't do anything to make her believe otherwise.
This went on for about 10 minutes, during which her breathing got heavier and heavier and her suppressed moans got a little louder. Then suddenly she grunted loud twice and I could clearly hear how wet she was as she rubbed her pussy furiously. Her orgasm lasted about a minute and then she lay still.

I was surprised that my little sister would masturbate knowing I was in the room, but I didn't mind her doing it. I'll admit, hearing her pleasure herself turned me on quite a bit and I was curious to see if she would do it again tomorrow night.

Long story short, I found out that my little sister likes to masturbate quite a lot. She did it every night, always waiting for about 20 minutes after we'd gone to bed and then she would start playing with herself.
Sometimes she did it in the morning too. She's a pretty heavy, long sleeper while I'm usually up much earlier. So on some days I got up and let her sleep and when I came back later she would still be asleep, only now she either had one of her hands down her panties or a pillow between her legs while rocking slightly against it.
It wasn't really masturbating because she was still very much asleep, but I found it funny that she did this unconsciously.

On the last day of her stay with me I did something that I never thought I would do before, but my curiosity (and pent-up arousal, probably) had gotten the better of me.
It was very early in the morning and I knew my sister wouldn't wake up for another couple of hours. I went over to the bed and carefully pulled off her blanket. She was lying on her back, wearing a white t-shirt and yellow panties. I shook her softly to make sure she was asleep and then, when she didn't respond, I slowly pulled her panties to the side to reveal her pussy.
She had pretty small lips that didn't hang out much and a surprisingly large clitoris that was only partially covered by the hood. None of the women I had been with had had a clitoris this big and I couldn't help wondering if it was so big because my sister played with it so much or if she played with it so much because it was so big. Anyway, I thought it was really hot and I would have loved to touch it, but I didn't dare to. It would have been really bad if my sister would have woke up then, so I just pulled her panties back and covered her up with the blanket again.

As I said in the beginning, this happened many years ago and to this day my sister has no idea that I heard her masturbating or that I looked at her pussy while she was sleeping.