Here 'crown' is used as a verb meaning to be the final triumphant act that finishes a long effort. So the final event to 'crown' the goodness of the country should be to maintain an attitude of brotherhood - i.e. treat everyone like a brother.
'From sea to shining sea' refers to the fact that the United States encompasses a geographic area which stretches from the Pacific Ocean to the Atlantic Ocean.
Student or Learner