The military absolutely has procedures for this - your brother won't just be stuck overseas if something critical happens. Most militaries (including the US) have emergency leave specifically for deaths, serious illnesses, or family crises. The catch is logistics and notification - someone at home needs to contact his command through the right channels, usually the Red Cross if it's immediate, or his unit's family readiness officer. The military doesn't monitor civilian news, so your parents or whoever is closest need to actually report it.
Once the command gets official notification of the emergency, they can process emergency leave pretty quickly - sometimes within 24-48 hours depending on his location and how remote the deployment is. He'll likely get called in, briefed, and put on the next available flight home. Travel time varies wildly though. If he's in a support role at a larger base, it moves faster. Combat zones sometimes have more restrictions, but even then, genuine family emergencies usually make the cut.
The real thing to figure out now is whether your brother knows how to report emergencies through his chain of command. Have him look up his unit's family contact info or point his chain of command toward the Red Cross emergency notification system. That's the official route that actually works. Your parents shouldn't just call the base and hope - they need to go through proper notification so it gets documented and reaches him fast. Better to ask about this now while everything's fine so everyone knows the process.