This is not exactly my belief but more of my sister's:
When she was really young, she thought that a person's sibling is the one they will get married to.
A childhood belief about family

others believed things like this
- i used to think that if you were a boy your younger sibling would have to be a sister and vice verca (sp?)— ben
- When I was young I thought that Jim Carrey and Mariah Carey were married because they had the same last name.— Aeris
- When I was very young, I thought that God was married to Mother Nature, and that together they made weather.Anon
- I used to believe that having children could only happen after a wedding with the person you are married to.Anon
- My younger sibling thought that Brittany was the name of the country where British people livedAnon
- When my sister's father went to Seattle, my sister said "Is Attle nice?" She thought Attle was a person's name— zzo38