This is SO sad! I think you DO need to rip your sister a new one.
We appreciate what you do for rescue. It is not for everyone. When I was that age (older by a few years even) I will have to admit I did something similar. The way I behaved towards my cat still haunts me to this day. I torture myself with guilt over him now, that I donate much of my time to rescue now. Hopefully your sis will grow up someday and understand what happened and do the same. However much I do, I still don't think it was enough for what my kitty had to go thru.
