Just came upon this rather wonderful quote by Lucasta Miller:
Facts, then, can become mythic through the way in which they are packaged and perceived. That there were three – and not four or five – Brontë sisters is, for example, a historical fact. But the motif of the three sisters has a cultural mystique stretching back into fairy tale, which unconsciously – or consciously, as when Ted Hughes calls them the ‘Three weird sisters’ after the witches in Macbeth – contributes to the sense of mystery which surrounds them. The historical accident by which three of Patrick and Maria Brontë’s five daughters lived beyond childhood, grew to develop literary talent, and become famous as a trio melds at some level in the cultural consciousness with the atavistic magic associated with the idea of the three sisters.