I read a story about a brother wrestling with his older sister and of course one thing leads to another.
The particular thing about this story is that while wrestling one day, his face ends up in her crotch and she grows quiet.
It's been bugging me since forever, I can't find it with any search terms.
I don't know if this is it, but I have a story that matches that I posted on another site. I reposted it here:
https://storiesonline.net/s/60381/my-horny-sister-by-bob-banger
I believe Lubrican also has a story with two versions that would match your title, but not your description. His stories deal with the sister taking rape defense class.