Something just doesn't ring true here. This lady says she doesn't work, but just to live and pay rent or a mortgage, plus food, transport, clothing etc., somebody is footing the bill. Maybe she actually also lives with her mother.
She says her mother will not allow her to sell the horse, until she pays the money owed. Why not sell this horse, who apparently jumps 6' and should fetch much more than the money owed? This way, mother could be paid from the proceeds and our OP would have quite a lot of money left over.
I absolutely understand the OP not wanting to part with her horse. It is always difficult to sell a beloved animal, but in this case, if in fact everything we've been told is true, it might be the only way out. If sold, at some point in the future, she might even be able to purchase the horse back.
If in fact the OP does have some kind of an income, then she could possibly find a new yard, pay the month in advance or whatever is required and move the horse when her mother and sister are not around. Not the best idea in the circumstances, but it is her horse and at 27 years old, she should be able to take charge of the situation and do something.