All kids are different. Some can be left home alone at age 8. Some shouldn't be left alone at 18. Let's give this woman the benefit of the doubt that her kids were in the former category.
As for the firearms, I literally cannot believe some of the things I've read.
My father was a MA state cop. There was always a loaded firearm in my parents' house growing up. Usually several - Dad left his BUG loaded as well (this was pre-1998, so no laws broken).
From about the age of, oh, 3 or 4 I was taught how to unload - and load - Dad's gun. I was shown that it was a tool, a dangerous tool that demanded respect, but a tool around which to be careful.
Nothing more, nothing less.
I never shot my sister. I never shot any of my friends. I was left alone - or with my sister, who's five years younger than me - from the age of about 12 or so.
It's about training, to be certain - you absolutely have to invest the time in training your kids how to handle guns safely, and many parents who are gunowners do not do this any more.
I do.
My son and daughter know the four rules by heart. They know that daddy's guns are not toys; that they are dangerous tools that must be handled carefully in the presence of a grownup. They know that if they find a gun somewhere, not to touch it, to leave the area, and to tell an adult.
They are no more in danger around an unsecured firearm than around a sharp knife, baseball bat, or washing machine.
As for the OP's story, WTG kid. You saved your sister's life and your own. The only criticism I can offer is to bring enough gun next time (kidding).