What does martin luther king mean by "I want to be the white mans brother not his brother in law."?

Dr. Martin Luther King, champion of the Civil Rigts movement in the 1960s, wants to emphasize that he envisions a US society in which there's full and true racial equality between the African-Americans and other fellow Americans, particulary with the white majority. He means that there should be brotherhood, that black people are no second-class people in the country, that they have the same rights and the same status others do have. Dr. King wants to see all together as in a close family, not the black person being a distant member, a "brother-in-law."