docs: def siblingsId(self) -> Tuple[CtsReference, CtsReference]:
          """ Siblings Identifiers of the passage
  
          :rtype: (str, str)
          """
  
          if not self._text:
              raise MissingAttribute("CapitainsCtsPassage was iniated without CtsTextMetadata object")
          if self._prev_next is not None:
              return self._prev_next
  
          document_references = self._text.getReffs(level=self.depth)
  
          range_length = 1
          if self.reference.is_range():
              range_length = len(self.getReffs())
  
          start = document_references.index(self.reference.start)
  
          if start == 0:
              # If the passage is already at the beginning
              _prev = None
          elif start - range_length < 0:
              _prev = document_references[0]
          else:
              _prev = document_references[start - 1]
  
          if start + 1 == len(document_references):
              # If the passage is already at the end
              _next = None
          elif start + range_length > len(document_references):
              _next = document_references[-1]
          else:
              _next = document_references[start + 1]
  
          self._prev_next = (_prev, _next)
          return self._prev_next