Saw my sister masturbate

Many years ago I came home earlier then usual that one day. No one else seemed to be home yet so I just went to my room. On the way I heard noises coming from my sister's room. The door was partially open so I took a look and saw my sister lying on her bed. She was naked from the waist down and clearly masturbating. I had a clear view of her pussy and was surprised at how big her clit was. She was rubbing it with her fingers and was moaning softly. Her eyes were closed so she never saw me standing there with my mouth open, watching her. A minute or so later she had a loud orgasm and I quietly went into my room, afraid that she would catch me watching otherwise. Needless to say I jerked off immediately after I'd closed the door behind me.

I've never been into my sister, nothing else has ever happened between us in the many years since and she has no idea I caught her masturbate, but I've never forgotten about what I saw that day.