Teaching their son that it's ok to take other peoples things? That couldn't possibly go wrong. Your parents are failing. You say you don't want to make your life a living hell but imo living under a roof with people like that, that you're supposed to be able to trust, is already hell. It might seem ok now, but given their doubling-down on defending your brother I suspect that in a few months you'll feel very differently.

Wait for the next time he puts his phone down outside the room, nab it, and pretend you've sold it.