Find a missing number

Little Pete came home from the playground with a bloody nose, black eye, and torn clothing. It was obvious he’d been in a bad fight and lost. While his father was patching him up, he asked his son what happened.
“Well, Dad,” said Pete, “ I challenged Larry to a duel. And, you know, I gave him his choice of weapons.”
“Uh-huh,” said the father, “that seems fair.”
“I know, but I never thought he’d choose his sister!”