By chance I happened to notice, in one of my beginner's books about how to learn Japanese (ref. Japanese), the following explanation for the signs of 'mother' (haha) and 'father' (chichi):
These signs are originally Chinese, of course. The doublets (haha / okaasan and chichi / otoosan) are needed in order to show more or less respect. (Respect is shown by o- and -san and different words entirely for other people's parents.) The strange pointing gesture of 'mother' caught my attention. Then I suddenly realized one meaning of an Eskimo myth, a myth with sister sun and brother moon involved.