soul sister

"Hey, Al, guess what: S is coming over with her dad, so I can take photographs of them for their holiday cards."

"She's coming over today?"

"Yes!"

"Awesome!"

...

"You know what, Mom?"

"What's that?"

"Sometimes S and I pretend we're sisters."

"Well, you know what, Al?"

"What?"

"There's something called 'soul sisters,' or 'soul brothers.' It's when you meet someone, and you feel so connected and close to that person, she feels like your sister, or he feels like your brother. That's your soul sister, or your soul brother. It's a very special thing."

"Mom?"

"Yeah?"

"S and I are soul sisters."

"That's very cool, Al."