Wow, Rideordie, I am so sorry this is happening to you and your sweet horse.
As others have already said, a family like this is toxic. I have dealt with families like this that, even with video evidence, still denied what their child had done. In one case, where a teenager forged the parents' signature, the mom told me it was none of my business (as a school official), and I had no right to impose consequences. For some parents, it is more important for their child to like them than to be good people and raise a good citizen. Get as far away from them as possible, even if it means moving to a new barn, because it will not end there.
I think, however, it is most likely that the BO will ask them to leave, as other boarders know what is going on and will be very upset if this is not handled correctly.
I also want to compliment you on how well you have handled this situation. You are quite an impressive young lady, able to take in lots of different advice, keep you head level, and protect yourself and your horse. Your family must be very proud of you!