Going by the picture I say she only had a couple hours left until birth.
There's usually a ten minute window for the babies to make it out alive after the mother dies - I'm surprised they didn't make it..
How did your vet try reviving them? Did he try beating on their chests at all?
We lost one of our does about two years ago in February from ketosis (ketosis and pregnancy toxemia are the same thing), we too had the vet out. He gave her a drench and said that was all he could do. He said its 50/50 and left. I went out later and she was already gone. We did a c-section to see if the kids were still alive but it was too late, everything was cold. She had triplets, 2 boys and 1 girl. She was due in about 2 days. That week the temperature was in the negatives.
Since then we never breed for winter babies, always April and after just because our winters are so harsh. Since then we are very careful during their gestation. Only the best quality hay we can get, regular supplements the last two months of gestation, minerals 24/7, as well as an herbal blend specifically for gestation.
Right now your main concern is your other ewe. Get some molasses and/or Gatorade/electrolytes and probios in a bucket of warm water for her. Plenty of hay, extra food, a warm stall in the barn, the usual. If you have any Calf Manna that will help too. Also see if you can get NutriDrench or an ovine equivalent(not sure if NutriDrench has copper in it, don't think so but make sure there isn't).
Do you have any lamb coats for the babies when they come?