What is the ruling on dealing with a brother who antagonizes his committed brother and ridicules him in front of people for practicing the Sunnah and attending religious lessons?

It is Allah's way to test and try humans in this world. People mocking you because you adhere to your religion is a sign that you are on the truth, for the polytheists used to mock the believers. Your adherence to the Sunnah of the Messenger of Allah, such as growing your beard, attending gatherings of goodness, and keeping company with the righteous, are matters Allah has guided you to. Do not be the first to be imprisoned for your religion, as some prophets and scholars were imprisoned. Deal with your brother with wisdom, advise him, be patient with his harm, and remain steadfast in your righteousness.