Ummmm, I dunno. There are probably more nonbenders than benders, I guess. I dunno if having bender parents makes it more or less likely. My Uncle Bumi isn't a bender and my Aunt Kya is a waterbender and my daddy is an airbender, and my grandpa was an airbender and my grandma was a waterbender.

A long time ago all the air nomads were airbenders, every one of us, but that's not it any longer.

And yeah. A long time before I was born, the Fire Nation killed all the air nomads. Except one.