What Exactly Constitutes a Sister?

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My 2 year old is wrestling with pesky pronouns and toddler taxonomy.

For months my son thought Sister meant any female person below the age of 20. I've been working on this with my son as it disconcerts the teenaged girls whom he embraces. -Which is to say, any teenaged girl that he comes across.

Next Sister came to signify any female person living in our home.  Unfortunately, I muddied the issue by having a female exchange student from Switzerland come live with us this summer.

"No, she is not your new sister.  Which is a good thing, considering that you are obviously intending to marry her."

At the same pool where last summer my son discovered Barbi, he recently worked out a new definition for sister.  It took some mental gymnastics for me to figure out why he was introducing the very buff, very muscular, and very confused male lifeguard as his new sister.

Then it hit me, a sister is a person taller than yourself, but younger than mom, who spends time bossing you around.

Basically sound logic.