With what you have, and especially with the papers, and not having signed them over to your sister, and having a bill of sale, you definitely have a case. The pictures really aren't going to do much though, as all they will prove is that you either owned him or rode him at one point, but it isn't prove of ownership. But it won't hurt to show the kind of relationship you two have. I just can't believe that your sister would do that to you. Especially theoretically selling him for 500. Not to be mean or weird or anything, but selling him for that little, she obviously didn't make any sort of profit from having him at her place, and feed, if she fed him, and any care she put into him. I could see a reason for selling him for a pretty penny to make some money off of him, but what she did just seems quite petty, and like she felt she had a bone to pick with you or something. I am so sorry that she did this to you, and I really hope that you are able to get your horse back. I couldn't imagine how I would feel and what I would do if someone, especially a family member did that to me.