I adore the twist. Wonderfully dark and perfect, the stuff of nightmares. The daydream is amazing, and the descriptions of both Narcissa/Sirius and Bellatrix are lovely.
They seem almost like children sitting down to a pretend game of tea party, until one little girl says something offensive, and there would be a bossy little girl who would throw down her scone and pout in her little dress covered in frills and bows.
I wish I knew who you were, so that I could read everything you've written.