What is the ruling on a brother severely beating his sister, leading to a concussion, not for a reason that displeases Allah and His Messenger, but simply because he is a man?

Assaulting others is forbidden oppression. In the Hadith Qudsi, Allah says: "O My servants, I have forbidden oppression for Myself and made it forbidden amongst you, so do not oppress one another." What the brother did to his sister is a great injustice and a crime that deserves appropriate punishment, so that it may be a deterrent for him and for others, especially since the Prophet, peace and blessings be upon him, enjoined kindness towards women and forbade assaulting them.