The not letting your sister there without parents was probably an insurance thing, I ran into something similar at a friend of my (now ex) boyfriends house – they had horses and I was “too young” to be allowed near them since their insurance wouldn’t cover if I got hurt (like I would sue a friend for their horse biting me?). But the rest sounds horrible mocha.
Important thing is to cover yourself in the contract – outline exactly what you offer, what you don’t, make it CLEAR that any “extras” of care (vet calls, etc) will come out of the owners pocket and you have full permission to treat a horse. Also, make it clear that you reserve the right to refuse to board an animal (like, ones that potentially have a contagious illness). I would also be wary of the fact that people tend to get a bit free with other animals on the property when they board their own pets sometimes – I have had people give wrong foods, etc to my animals, or try to pet an animal that is known to bite even after being warned, when visiting their own pet when I let their animal stay with me.