What do you do?

[kurosh]

I wanted to save my carpal tunnel post for something special, but I couldn't think of anything so first, yay, carpal tunnel!

You're 19-22ish and live near your parent's house. You are big and strong. You have a younger brother, 16, and a younger sister, 14. You like them both. Your parents are out of town for the weekend. Your sister has been raising money for one of her friends, who has recently become very sick and needs it for hospital bills.

She calls you and tells you:
The $1000 she has raised so far has been stolen from her room. Your little brother is having a party that night with 9 or so of his friends. They've been drinking and doing some drugs. All of his friends are still there. You're 99% sure one of them stole it.

What do you do? Can violence be avoided with a positive outcome here?