Complex sentence: The largest Cathedral by area is St. Peter's Basilica which is also the longest or Basilica of the National Shrine of Our Lady of Aparecida in Brazil, depending on how area is defined.
Split into simpler sentences: The largest Cathedral by area is St. Peter's Basilica which is also the longest. An alterantive contender for largest Cathedral is Basilica of the National Shrine of Our Lady of Aparecida in Brazil, depending on how area is defined.