Knowing the direction RWBY is going, Cinder's not going to get backstory until the episode before she makes a heel-face turn, the episode she dies, or the episode where she does the former immediately before the latter.Sounds almost like the greentext story of the Bethesda "Dreamboy" employee.
Cinder is a good example of picking up a cool concept, creating a character that by all means should be good and doing fuck all with it. Her backstory is, as you correctly noted, long overdue, with volume 4 being the best opportunity to show it. Since the bitch is based on Cinderella, Miles could have used that fairy tale as the backstory, but unlike Cinderella, Cinder never got her prince and Lionel Richie never dedicated a song to her. But Miles is a galaxy brained nibba and I'm just some random faggot, so what the fuck do I know?
Why Cinder hasn't been killed off a long time ago, I do not know. I speculate that Nigiri might be a factor. Then again, Monica Rial was only a brief unpleasant memory with the very few voice lines Sienna Khan (another wasted character) got.