My brother wants my girlfriend, what should I do?

He flirts with her, puts an arm around her, hugs her, grabs her ass. He tells her to leave me for him. He even tried to get her into his bed at times.

I don't know what to do. He has kissed my ex and slept with her in the past. What do I do about him?
My brother wants my girlfriend, what should I do?
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