Consult the police and the investigative detective about your concern and circumstances, subdue your brother with some help of grown men. Take him to the hospital for DNA test.
Don't discuss this at home. Take your parents out of the house for the night, go straight to public place or police station, and discuss it there. No need to make sure your "brother" is at home at all during your discussion with your parents and the police. If your brother is still home by the time you finish talking to your parents, he is still your brother. If he is gone, that means he is the serial killers and already suspecting he has blown his cover.