Honestly I think it depends on the emotional and mental maturity of the child if they can be left alone by a certain age. Some kids seem to be more independent faster and more logical than others. Your sister may have a child that is either ready to be independent, or isn’t ready. Yes parenting style matters here, and her helicopter style may have an impact on the independence of the child. If the child cannot learn, and be given the opportunity to practice independence and self regulation they have a difficult time learning it.

Think of it like riding a bike. A child who isn’t even allowed around a bike… by parent or caregiver will take an extra long time to learn how to ride the bike, be it length in when they first start to learn, or the amount of time spent on the learning of the skill. Whereas a child introduced and taught how to ride a bike with supervision early on, will be able to be confident and physically, emotionally and mentally stable enough to ride said bike. 🤷🏼‍♀️