Why I'm Making a Story About Sisters When I'm an Only Child

I have no siblings. Growing up with someone around my age in the house is totally unfamiliar to me. Still, I like creating stories with characters who have a brother or a sister or both. My current project is about two sisters who live together. It's inspired this strange conversation that my husband I had about what life would be like if he was my sister or if I was his brother.

Anyways, here's a note/sketch of why I find characters with siblings, especially sisters, so interesting:

On the other hand, many people find the fact that I don't have siblings interesting.  Yes, I'm a little spoiled.