Being a citizen in a civilized society involves engagement in the social contract. Your brother and mother seem keen to return to pre-law anarchy of some kind. If finders are keepers where does it end? If she leaves her car keys out is the car now yours? Can you sell her silverware if she doesn't lock it up?

I do understand your desire to not rock the boat and your safest option is to begin planning for your exit from this environment. Unfortunately their are plenty more unscrupulous people who will not hesitate to take your things so be careful who you trust.