I'm not really sure If I can explain it well but in general, their relationship is something similar to a junior-senior kind of thing. The blonde one (Silver) watches over the other and helps her when she's in a pinch while making sure she won't step out of line and break the rules. In case that happens, Silver is prepared to take drastic measures. At the same time, the one with the black wings is prepared to fight back if she deemed an injustice in their rules.
You could read my webcomic (although the series is not entirely finished yet) : tapastic.com/series/tabw
if you have the time.
Thank you for the comment! I absolutely appreciated it!