I remember your initial post about this months ago. As I said before its such a difficult situation to be in, especially when its family. I know you are going to find this hard, but you're going to have to go to town on them. Report them, not just to the Sherriff's Dept, but to some animal cruelty organisations. You've tried to help them/advise them and they've just thrown it back in your face. Bite the bullet as the saying goes, and do what you think is right for those horses, before they're too bad to save.
I watch a program over here in UK about Animal Cops Houston!?! Do you have any place like that near by?
I'm sure they wouldn't just stand by like the Sheriff does, they'd get the ball rolling and get those poor babies sorted.
I'm sending you (((BIG HUGS))), and you know how to reach me if you need a chat.