Twins are so difficult, sorry the filly didn't make it. My only experience was with a friend's QH mare years ago. The smaller of the two colts was actually the surviving twin - a bigger, correct looking colt was delivered dead and the one who survived was stunted and club footed. She kept him all of his life, I think he got to maybe 13 hands or so. I had bred my mare to the same stallion that year, got mine ultrasounded and she didn't....
Jan