I don't know about books for sibling boundaries.
I'd just tell her you don't want to discuss her sex life. If she persists, it's time to end the conversation.
I'm far from being a prude and talking about sex openly doesn't make me blink. But there was an issue when my kids became adults with boundaries such as this. too much information I set up the boundary. I was firm. Unless it was a specific question or whatever.......there are just some things a mother really doesn't need to know. The same way I wouldn't discuss bedroom activity between their dad and I with them.
