I dont know what to do

You have your answer right here.

If your sister is that bad, why would you even board at the same facility? If the BM is so ineffectual that they have no clue who isn't being fed properly, why would you continue to board at that facility?

Its the only place I have to go. If I could pack up and leave with my horse, I'd be long gone.

I don't know how the BM will react because I haven't contacted her yet. I'm trying to have actual proof that my sister isn't taking care of her horse.
Posted via Mobile Device

If your description of the horse is correct, then his appearance is all the proof that is needed.

Take pics to show the horse's condition and the state of the stall. You worry too much about what's not important. Don't worry about your sister doing something to your horse as she knows she'd be stepping over the law and could be in serious trouble.