My experience was not with a sister but a girl cousin. I was 12 and she was 7. Our family would gather at our grandma’s every weekend. Not sure why but she would sit on my lap all the time on her own accord. I remember I would get a hard-on everytime she sat in my laps.
Everything started on the day when she reached down between her legs to feel what was pressing onto her bum. I got so aroused by her meddling with my erection that I started having wild thoughts in my mind. I took a throw cushion on the couch and placed it on my cousin’s lap while she continued sitting in my laps, and started running the tip of my index finger lightly over her privates through her pants. This became a secret between us and a routine. We both kept hush about it without ever needing to tell each other to.
The thing that got me to think that she seemed to enjoy it was that ever since the first incident, she would be wearing a dress everytime we gathered at my grandma’s, or other relatives’ or when we visited each other’s homes, when previously she would be mostly in pants/shorts. She seemed to know the ease of access if she wore a dress. And so this secret petting affair we had continued without anyone else noticing somehow. Another thing that got me to think she enjoyed it was that she would get very wet/slimy and she would close her legs tightly together before opening them up again whenever the touching went on for a while. What begun as touching through clothes, soon escalated to direct flesh contact as my curiosity got me reaching under her panties. Something else I also noticed was how her clitoris would enlarge to a hard bulb from just a flab of skin. As a young boy at puberty, all these turned me on really badly. She would also take my hand and place it at her privates whenever I wasn’t touching her. So with all these gestures and bodily signs, I would often rub her gently but with varying speeds/intensities, till she jammed her legs tightly together and shivered/shuddered.
This continued till I was 15 and she was 10, I stopped because I felt really guilty/bad. We continue to have good cousin relationships till today.I will like to share if anyone or girl have same experience.