First of all, it should be clear that a Muslim should seek a righteous spouse for marriage. Religion and good character should be the top concern of every Muslim when looking for a spouse. Once you have chosen the person for her basic faith and sound character, you are doing the right thing. Your brother has no right to object   your decision to get married, but he has the right to express his opinion and he has the right to discuss with his family. Islam does not teach us that we marry only within our ethnic circle. Islam treats all humanity as a single family. So speak to your brother gently.