Had frequent sneaky sex with my sister when I was 10 and she was 9.
It was mostly under the blankets or hidden because we shared the same room with our smaller sister who was 5.

The best times were when my smaller sister slept with our parents. But overall, we did it for about 2-3 years until she got her period. After that, our mom gave her a scare with pregnancy and our relationship fizzled out. Especially after our rooms were separated. We never talked about it after that.

I'm 31 and she's 30 now