Horse needs to be fed and grained too. This amount of land won't sustain anything at all for much more than week, and by time horse walks it down?
Will be weed city.
And don't know where you are but grass is just starting to come up here, and if had a horse, even on 2 acres at this point, would still be feeding hay.
Basically, if horse has good grass and plenty of it, it will not touch hay usually.
IF the grass is not good or enough, it will devour hay all year around.
If your sister doesn't want to properly take care of horse, and situation you are describing is not proper care, then she needs to get rid of horse.
Horses make me a better person.