Nephew keeps going through my underwear drawer?

I agree with the answer above mine. I would let them know, while you agree to house them, and share the space, that, that space is YOURS and not to invade as to keep things peaceful and comfortable. I think that is best. I also find it a tad creepy, me personally, I would slap him upside the head with a dildo and maybe he wouldn't go snooping through any one else's stuff. But that's just me.