A childhood belief about family

When I was in kindergarten, my friend always talked about her cousin. One day I decided to ask her what a cousin was (at the time I didn't have any cousins, so I didn't know what it meant) and she said "it's like a baby brother or sister", so being only five or six, I thought it WAS a baby brother or sister. I thought it was a fancy/smart way of saying brother or sister. So throughout the rest of the school year, I referred to my younger brother as my cousin since I thought it would make me cool, until my mom told me that I don't have any cousins. I was pretty disappointed, and everyone thought I had a really cool cousin.