If I let my brother use my car under the condition that he would pay the note he but never did and he won't return the car, is it considered stolen?
Technically, it is: he is taking the vehicle when his permission to do so has been revoked. But in my experience, the police will not see it that way: because you initially gave him permission to take/use it, they will see this is a non-criminal "civil" dispute over the terms under which he may use it (e.g for how long; having to do or pay what in exchange for its use). You most likely will have to sue him about his: you can sue him for a court order for its return, or simply for the value of the car and any other amounts he owes you (e.g. for its use, such as the note payments during the time he had it).