How to get my sister to stop watching this guys pitbull(she leaves him with ME)?

so my sister is watching her ex bf's pitbull while he is away in the army. she is NEVER home and leaves him with me and my fiance! we noticed something was off with him, and after MONTHS of convincing her that he is sick, she had the guys sister(who was watching him) take him to the vet. now he has to have both hips replaced and his leg fixed. the guy refuses to let the surgery happen until he is hope in september. so he has to stay HEAVILY medicated and who has to give him these med? WE DO of course! and by that, i mean not my sister! I told her that my fiance and I discussed it, and the dog as to go back the the guys sister (who gave hi to us to watch bc she works alot). and she said I have no choice. we are staying in a house my father bought us to share. what can I do to let her know I am serious and he needs to go? I cant medicate him all the time and take care of him, I work too and she doesnt help.