Sometimes, I think that people make these combinations by just rolling dice against a numbered list of characters.
So, Mystia has the power of song and darkness/blindness, while Remilia has fate and vampirism.
Song of fate?
Hmm... maybe I should think of how Touhou children gain powers more along the lines of something like Touhoumon, so Mystia is a Normal/Flying type, and Remilia is a Dark/Flying type, so she should have Dark/Flying type powers?