They make you laugh, drive you mad...and will be your friend for life.
Brothers and Sisters are among the most important people in the world to us.
So this week on Newsround we'll be celebrating everything about siblings and finding out from around the world what's different about being a brother or a sister - and what we all share.
We wanted to find out what you think so asked a group of more than 1000 children aged 7-12 to tell us all about their experiences.
Here's Hayley with more.