Contact your homeowners insurance company, and determine if your insurance will cover the bill.
All things considered, $2,500 is not as bad as these claims get.
It's true, the brother was not being too smart, but if you have a dog that has the potential to bite the way yours did, you should have secured him better (kennel, tie-down). This is one of those situations where, in "legalese", it was foreseeable that the dog might bite a stranger in the house, and once you permitted the stranger and his whole gaggle of tag-alongs in (even though you didn't really expect them), you are expected to do what you reasonably can to keep your guests safe, knowing that between the lot of them (and the kids), someone might open the door to the room where your dog was lose.
I'm sorry my answer is not more sympathetic, but, as I said, $2,500 is not so bad, and your insurance might cover it.
I hope I addressed all of your question and I've helped. If so, please
accept my answer.
This is not legal advice, read my
disclaimer.