The most important thing is to let them dry completely before touching them or trying to separate them, so put them in a warm, dry place without direct sunlight for a few days. The ones stuck together look like they might be saveable if you act fast - sometimes with patience and a hair dryer on low heat they'll separate without too much damage, though some might end up with marks. For the future, store them in individual protective sleeves (the kind they use for comics in shops) inside cardboard or plastic boxes with silica gel desiccant packs, in a cool, dry place! Seriously, it's super frustrating what your brother did but not everything is lost if you act now.