I can't imagine anyone who knows anything about Japan thinking that shisutaa meaning nun, let alone ane or imouto, was or could have been current in Heian. My bet is on the Cute. In fact, it strikes me as an almost shoujo manga kind of cute: she's a *nun*, see, even if she's a Buddhist nun, so we'll call her 'shisutaa' sporfle giggle.

Unless it's a kind of Tolkienism: 'his name in my invented language is Blahblah meaning stay-at-home which I've rendered by the middle English hámfoest and so, Hamfast.' "Her name in Japanese means little nun which I've rendered as the English word sister pronounced as it would be written in katakana."