Doing stuff like that when she is on her own is one thing, but taking her brother with her is an entirely different kettle of fish altogether. You really need to come down hard on her with this one I think - take away her phone and her computer access for a few days because as you would know, they can still live a very full social life from home because everyone they know is only one click away. What she did was really irresponsible and possibly even dangerous - what would she have done had she got 'bored' with her brother? Left him there? Sent him home on his own? As Dr Phil would say, you enable their behaviour by letting them have access to stuff like computers and phones so taking it away is the only way to curb them for a while.