All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If your brother is as you mentioned, then he is doing wrong and is deceiving his parents. Please refer to fatwas 87856 and 85041.
You did well by talking to him about this matter. We advise you to continue admonishing him in a kind manner and remind him of Allah as well as the bad consequences of his actions in this worldly life and the Hereafter. He may ruin his future and regret when it is too late.
If he takes heed, then praise be to Allah; otherwise, threaten him to inform your father about it. If he takes heed, then praise be to Allah; otherwise, you first inform your mother, as she may convince him by stirring his emotions, and if it is necessary to tell your father in the end, then do so.
Allah knows best.