public function addSibling(EntityInterface $sibling, $position = null, $returnSibling = false)
    {
        if ($this->exists) {
            if (is_null($position)) {
                $position = $this->getNextAfterLastPosition();
            }

            $sibling->moveTo($position, $this->parent_id);
        }

        return ($returnSibling === true ? $sibling : $this);
    }