I don't think that you would necessarily need to wait for the kids to bring it up. This could probably be brought up in a couple of different conversations. I don't know that I would speak to both of them at the same time though. At 8 SD thought that babies were made by swallowing a baby seed. At 10 she knew all about the physical parts of sexual development and the pregnancy cycle and was just starting to learn about sexual relationships.
My SD11 says I have a lot of long boring talks with her.
But I have been trying to be upfront and honest with her about relationship issues (both friendship and boy/girlfriend issues). So one thing I would talk to her about is rumors. If someone says something you can't automatically believe it to be true. You should ask the person directly. (SD11 just went into middle school so this was a big topic). I believe we've also had a separate talk about the seriousness of making accusations that are untrue (DH was accused of sexually abusing SD so we have spoken to her about false accusations that can lead to very serious consequences like daddy ending up in jail and all the time wasted for the police and CPS to investigate a false accusation when children who are truly being abused are not getting attention). I would definitely stress that an accusation of sexual inappropriateness is a very serious accusation and should not be made lightly or used as a weapon.
And, since SD is at that age, I would have a conversation with her about inappropriate sexual advances or comments. We haven't quite gotten that far but with Trump in the news it is something we should discuss soon.