I just caught my brother wearing my panties again what should I do?

I have told my mother about this and she just told me it is a ping fad. I just know he goes to the hamper and takes a few pairs and a few weeks later they magically reaapear in the washer. What can I do about this? He is 15. I am thinking of inviting my friends over and then pull down his pants so all the girls can see his panties or should I say my panties!