How'd you find your horse? Just "looking" for it like driving around? Or did you find out who bought it? Did you go there and demand your horse's return? Did the police come with you? A lawyer? Come on! Details would be nice so we all know HOW this was resolved.
I must say, if you got your horse back in THAT horrible of a condition, then you need to call the ASPCA down there and report those people! Do they treat all animals like that? Was it your sister that did this to the horse, or the new owner(s)??
It's a crime what happened, not only that your horse was sold w/out your permission, but your horse was starved and mistreated, too...
Someone's heads should roll for all of this!
At least give your sister a beat-down like she deserves for selling your horse.