Sorry for your loss OP. Twins are always a risk and sometimes not even detectable with even the best vets. A well known breeder in the area had a mare checked and no twins, yet at birth a second one was discovered. Only the first one born made it. The second was too small and undeveloped. The positioning of one behind the other apparently made it difficult to detect. She had a top repro vet. I do as well- one who works with the Rohara mares and others on the breeding circuit, but sometimes they miss things- just like human doctors. I have them checked twice, especially if they come up "open."
Glad the mare is ok.