The kid in your group of friends that always talks about their younger or older sibling. No matter what the topic that person will either interrupt you or immediately when you're finished they'll say something like, "Omg! Me and my sister/brother do that all the time!" or "Really? Cool. Yeah my sister/brother had those pants but she/he gave them away once they were out of style."
John was such a sibling that he began to have wet dreams
of his younger brother, Billy.
Sam kicked John out the group since he was such a sibling. After all, John was always commenting on how hot his 5 year old brother was, and that was just plain creepy.