Your brother borrowing your car without asking once is a mistake. Twice is a pattern. Name it.

The first time really can be thoughtlessness, and treating it as a character flaw would be unfair. The second time tells you the first one never registered. Family is where patterns stay unnamed longest, because saying it out loud feels disloyal and everyone involved has years of practice at letting things go. Unnamed things do not stop; they just become normal.

Name it once, plainly, with the request attached: ask me before you take it, every time. No history, no list of past offences, no raised voice. If it happens again after that, it has stopped being a misunderstanding.