I went through a similar situation that you seem to be describing, with J having a brother, and parents struggling with what to project to the brother if people are allowed to sleep over for me.

They had to redefine the rule to relate to pregnancy, and then it made sense. I'm also older than him, and that played a part. I guess subconsciously that is a large part of why sleepovers aren't allowed at younger ages in general. On that note, at the same time it is best to let young people have a safe, trusting space to do things, if they're going to be doing them anyways.

This is tough. To not allow her at sleepovers or to have sleepovers could negatively impact her social life, and encourage negativity on her part in regards to effects of her sexuality. And about being honest with you. As an only parent it'd be very harmful to have communication and trust damaged.

I'd say make the sleepover aspect, in terms of equality for brother and sister, a pregnancy thing. She can have people sleep over because no one is getting pregnant, and she is X age (hopefully she is older than him). Even with a condom there is still that chance. Alternatively, my mom transitioned into allowing it by having us sleep with the door open. We respected traditions enough we never did anything, and it was brownie points if they ever peeked and saw us adorably cuddling and happy.

Some things to think about. Thanks again for looking for answers, and feel free to PM me if you have any questions or need anything. You're a standup pops for wanting to learn for your daughter.