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