Should I lock my little brother out of the main house until our parents get home in four days?

I have to watch my 13 year old brother and almost 6 year old sister while my parents are out of town. My brother is usually demented but he's been worse with our parents away. He has been downright dangerous too because I caught him making mustard gas and he was going to poison our sister's bird menagerie with it. He also turned her favorite doll into a voodoo doll and wrote what he says are "curses in a demon's language" in all of her favorite books. His room is in the pool house and has been for a couple of years because my stepmother said she couldn't pray with all the voodoo under the same roof. She had tried to help him when he first started doing it but nothing worked. I am thinking I should just lock him out of the main house (make him give me his key) for the rest of the time my parents are gone because I can't handle the danger. I'm kind of scared though if I do that he might start a fire or something.