If I were in your shoes, I'd eat the loss, lay low, and get out. You have to look at the battle versus the war.

It may be tempting to call the cops, and you could certainly do this, but there's really no upside. The cops probably won't care much, and your family will gang up on you and make you look like an overreacting crazy person. No matter what the outcome, your brother (and by extension your parents) will be furious with you for throwing the cops at them in the first place. All you'll do is teach them to be sneakier next time about how they choose to mess with you.

See my main concern with this [...] is he obviously will register this as "keep going". So what will happen when he, more than likely, begins stripping away every little possession I currently own? [...] where is the line drawn here, before I literally get the small amount of things I own, just stolen from me from a GC?

That's just the thing, though: You're not operating from a position of strength, here. Your entire family has already decided that it is acceptable to treat you unfairly. If they can't steal from you, they'll do something else. There is no line to be drawn because no matter where you draw it, narcissists will find a way to step over it. You're trying to act in a way that "teaches" them to treat you better, but that's not possible with narcissists.

There's nothing stopping your brother from screwing with you over the long run. The only thing you can do is stay out of their way as much as you can.

It's not fair at all, but that's the best strategy. You need to focus on saving up enough money to get out from under their influence.