Hmmm, I have seen some other threads about help with this kind of thing (vet bill, emergency things), but I wouldn't know where to start to narrow down the threads
. Maybe you could check with your local humane society, or vet clinic, or rescues or do a search here in the SOS threads? They might know or you might find a list there...Good luck!
I am sorry your sister finds herself in this position - I agree with the above posters, check at the local shelters and humane societies - I am not sure which country you are in but some animal centers do offer free treatment pending circumstance of the owner .