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.