[I](https://imgur.com/a/24VJa2D) am 39 now so this was a while ago.
My sister visited one year not too long after I started dating my now husband. She was fresh out of high school and was about to start college a few hours from us. After a few days I could tell she was a little off. I asked her what was up and after a few minutes of her dancing around she finally told be she was having intense dreams of my boyfriend fucking her and experiencing her first orgasm from sex.
I laughed, why I do not know. I told her he was really good at that actually. For some reason I started having the thought of sharing him and it really turned me on. I asked her if she wanted to surprise him tonight and we can have a little fun with him. She thought I was kidding. I just walked away.
That night I was in my room, hubby was in the shower and sis came in. She asked if I was serious about earlier and I just nodded. He came out the shower in his boxers only, as usual and saw my sister. He turned white as a ghost and froze. He was trying to apologize but he kept fumbling his words. It was adorable actually.