Gtk.WidgetPath.prototype.iter_get_sibling_index

function iter_get_sibling_index(pos:Number):Number {
    // Gjs wrapper for gtk_widget_path_iter_get_sibling_index()
}

Returns the index into the list of siblings for the element at pos as returned by Gtk.iter_get_siblings. If that function would return null because the element at pos has no siblings, this function will return 0.