I really enjoyed this! Such strong characterizations: a layered and believable Millicent; a deliciously-grating Rita; a nicely-complex Sinistra. And OMG, an absolutely to-die-for Minerva/Aurora. I'll be rereading that picnic tryst many more times, I can tell you (silver waterfall of hair, curvy Sinistra...lovely.)
I also like your depiction of Slytherins -- their camaraderie, the notion that it's subtlety and resourcefulness, not "evil," that mark them, the way that Millicent and Sinistra are able to work together. Other things that please me (in no particular order):