No legalities from me, since I live in Finland. Just some observations from an older sister of four.
My brothers and I were left alone for a few hours since I was about eight or nine. Mom would go grocery shopping, but didn’t want to take screaming, fighting kids with her, so she’d leave us to play in the fenced and gated back yard. We lived in apartment buildings or duplexes for quite a long time, so usually there would be someone around whom we could go to if an emergency developed. We also had certain numbers preprogrammed onto the phone so that if something came up, we could call grandma, our aunt, or our neighbors.
Since I have four brothers in total, my mom trusted me pretty early on with things like baby-sitting in the evenings. Mom and Dad would go visit their friends or whatever they did, and they’d be back at around 11 pm, by which time hopefully I had gotten my brothers into bed and we were all sleeping.
I think the “peace of mind” thing might in some ways be easier attained if there are multiple kids. If you have siblings, chances are you have to help your parents take care of them in some way. This means that you eventually have to acquire skills like making simple lunches or changing diapers. I think this might be a way to ease qualms about whether your stepdaughter can handle being by herself. You could let her help you cook meals, for example, and make them simple enough that after a few times, she can make them herself. This way, you can stock the fridge beforehand with food items that she can prepare if she’s alone for the evening and gets hungry. A list of numbers by the phone is also a really good thing; if she knows that there’s always someone she can contact if something isn’t working out right, it will probably give her more confidence.
As to being left alone overnight, I can’t really say. My parents stopped asking aunts to come and babysit weekend-long trips when I was around 15. At that point, my elder little brothers were old enough so that they could pretty much work themselves out–go play outside with their friends, make sandwiches or heat something in the microwave for lunch–and I could concentrate on the one-year-old.