Complex sentence: They are one of the two remaining charter members of the National League (the other being the Atlanta Braves) and one of two active major league clubs based in Chicago, the other being the Chicago White Sox of the American League.
Split into simpler sentences: They are one of the two remaining charter members of the National League (the other being the Atlanta Braves). The Cubs are also one of two active major league clubs based in Chicago, the other being the Chicago White Sox of the American League.