When my guinea pig had fallen over and I fruitlessly tried to prep her up using popsicle-sticks, I realized she had died.
My three-year-old sister, prone to giving her stuffed animals bathes, stood above the paint bucket where our lifeless kitten float in the milky water. -true story
This is awful, makes me really sad, and if I were your sister yeah, I'd probably still get teary-eyed at least when it was brought up. For a long time (5ish years) I was on-and-off OKC. One of their match questions is, "Which is worse, child abuse or animal abuse?" I thought about it for a long time and had different opinions over the years but by the end I generally became inclined to say "animal abuse." Thing is, child abuse is awful - but a child has a much better chance of communicating with outsiders and potentially seeking and finding help than an animal does, generally speaking. There are established support networks for children, etc - basically, abused children eventually have more resources and more help down the line, assuming/hoping their abuse is uncovered and they are removed from the situation, or they reach an age/maturity where they can seek escape themselves. I think it is more likely a child will be saved from abuse than an animal will be. This article, which I warn you all is extremely upsetting, demonstrates the exception to this rule. Sadly. It's horrific and awful. At no point am I saying child abuse is okay or not bad or anything. Child abuse is awful, horrific, and should not happen. However, it seems to me that animals tend to be more completely helpless, isolated, and dismissed than human kids. (I may very well be wrong.) - not trying to say that your sister is an animal abuser at any point during this, the dichotomy is just part of what sprung to mind as I was typing my response. It is incredibly awful when an animal dies at the hands of a human and it's not intentional, especially in the case of your sister who was simply way too young to know.
Yeah, that's a bummer too. To this day, if it's brought up around my sister she cries.