I had a similar diet meltdown this summer, though for far less emotional reasons. For me it was a total kitchen rehab. We were living for three months out of a microwave and a fridge in our living room. And from there we devolved to eating out, and then to ordering out, and then right back to the usual SAD junk food diet.
Even though I knew exactly what I needed to do in order to feel better and start losing fat again, it was a monster struggle to actually do it. It was harder to let go of the junk this time than it was the first time! I knew -- because I'm familiar with my own wacky psychological quirks by now -- that if I could just get through one day of eating right, I'd be able to stick with it again. But getting that one day in felt almost impossible. Every day for weeks I promised myself "this is the last day. Tomorrow I get serious." And then tomorrow was just as hard to manage as the day before.
I finally managed to white-knuckle it through a day where I ate everything in sight, as long as it was primal. After that, things got easier again, and I've been doing well for a couple of weeks. But wow, was the "before" hard to end this time around!
One thing that I found helped was stocking my fridge and pantry to bursting with my primal favorites and just eating like a maniac from that for a few days. No matter what I wanted, I stuffed myself with bacon and eggs, or guacamole, or steak and broccoli. That was my answer to every craving. Eventually the cravings gave up and left, but it wasn't a lot of fun fighting them off.
I feel for you -- and your sister, too.