This is based on a statement in the "Kojiki" that Emperor Anko took his older sister, Nagata no Oiratsume no Ojo (Nakashihime), as his empress consort after he had executed Okusaka no Miko.
これは、古事記に大草香皇子を殺した後、妻にした皇后が同母姉である長田大娘皇女とされているためである。