Apparently they are named after characters from a game series. <a href="http:&#x2F;&#x2F;en.touhouwiki.net&#x2F;wiki&#x2F;Prismriver_Sisters" rel="nofollow">http:&#x2F;&#x2F;en.touhouwiki.net&#x2F;wiki&#x2F;Prismriver_Sisters</a>