def makeSiblingTemplatesList(templates, new_template_file,
                             default_template=None):
  """Converts template paths into a list of "sibling" templates.
  
  Args:
    templates: search list of templates (or just a single template not in a
      list) from which template paths will be extracted (discarding the final
      template file name of each template)
    new_template_file: new "sibling" template file to append to each extracted
      template path
    default_template: a default template (or a list of them) to append to the
      end of the generated "sibling" template paths; default is None
 
  Returns:
    A list of potential "sibling" templates named by new_template_file located
    in the paths of the templates in the supplied list.  For example, from:
      ['foo/bar/the_old_template.html', 'foo/the_old_template.html']
    to:
      ['foo/bar/some_new_template.html', 'foo/some_new_template.html']
  """
  if not isinstance(templates, (list, tuple)):
    templates = [templates]

  if default_template is None:
    default_template = []

  if not isinstance(default_template, (list, tuple)):
    default_template = [default_template]

  sibling_templates = [
    '%s/%s' % (t.rsplit('/', 1)[0], new_template_file) for t in templates]

  return sibling_templates + default_template