You know... I did say it was your sister's fault and I stick by it.
There's a responsibility on the part of an owner to act in a manner which keeps their animal from becoming a danger or succumbing to a danger and your sister was negligent in this duty. It was a mistake, they happen... but it was HER mistake and it's upon HER head the guilt and culpability must lie.
Incidentally I would and have said similar things to anyone who has less than ideal practices with ANY animal. Snake got loose? Who's fault is it for not making sure the enclosure was secure? Lizard died? Well you shouldn't have been a F!@#$%in' idiot and caused malnutrition.
Your story about your nephew crying himself to sleep every night is very touching and loaded with human interest heart tugging but it does not change the fact that the owner of the animal was negligent in some small facet of their care and their animal suffered for it. It's easy to blame the evil neighbor who may or may not be doing this deliberately... How about putting the blame where it really belongs though? On the one who let the cat outside where it came into contact with something poisonous.
"Genes, Like Leibnitz's monads, have no windows; the higher properties of life are emergent... And once assembled, organisms have no windows." - Edward Wilson, Sociobiology