Huntingirl, this is just awful, but something just seems amiss. Does your sister have issues? I find it hard to believe that this is the first time she has been difficult like this. (you know we all tend to follow patterns) I hope that you get your horse back. You have had all the right suggestions. I just have a hard time imagining taking a horse all th way to MO to turn it out for a while because it was acting up. However it probably made sense at the time and you had no clue she was going to be so crazy. Do you have any family members that could intervene? Willl she tell you who she sold it to? Perhaps you can reason with them? How about her husband or friends, anyone who can find out why she is acting this way?
Please let us know how it goes.
P. S. I just realized its your half sister, can your parents get involved? If you can find out who she sold him to you have a good chance of getting him back.