A childhood belief about marriage

When I was a little girl, like four years old, I thought you could marry anyone you wanted, including your siblings. I thought it would be easier to marry a sibling because you had known them your whole life, you knew everything about them, and you wouldn't have the trouble of getting to know somebody new. I only had sisters and no brothers, so I assumed I was out of luck, but my friend had a twin brother. I said to her, "When we're grownups, why don't you just marry him?" She explained to me that you don't marry siblings. I'm lucky she knew that, or I wouldn't have cleared it up at an early age.