What should I do regarding my brother who looked at my older sister while she was sleeping with a specific intent, then confessed to me that he repented, and should I tell my mother, taking into consideration that he trusted me?

If the brother looked at his older sister with a forbidden gaze, this is a grave sin, and he erred by telling you. As long as he has repented and asked for discretion, do not tell anyone. You can threaten him by telling your mother if he relapses. We advise you and your sister to be cautious and vigilant, not to adorn yourselves in front of him, to close your rooms when sleeping, and if any suspicion arises from him, you must treat him as a stranger, veil yourselves from him, and avoid being alone with him.