For this approach, there's three ways to deal with it. The Good way, Bad way, and of course the Aleister Way.
Good Way - Be calm about it, give a small lecture about your privacy, inform your parents, try to peacefully settle the situation
Bad Way - Scream yell, throw things, yell at the parents, blame everyone else
Aleister Way - Walk up to them whie in the room, seem calm, then knock them the hell out, drag then into their room, revome all their clothing, if they have a cell phone put all the contacts as "911"
I, however, am an only child and have good privacy.