18-year-old forbids 13-year-old sister from coming over to her house after she cut her doll's hair: 'She’s old enough to know right from wrong and this was unforgivable.'

If a 3-year-old cuts a doll's hair, you could argue that they didn't know any better. There is no world where a normal 13-year-old doesn't know that it's wrong to damage her sister's property. 

One of the greatest inventions in the history of humankind is the American Girl Doll Hospital. American Girl Doll Hospitals are open at American Girl Doll Stores and provide essential services for dolls who have been put through the wringer. If their owner has never brushed their hair, they can fix it to make it look as good as new. If a little brother has drawn all over the doll with a marker, they can flawlessly remove all signs of damage. It's a wonderful resource because it's free. American Girl Dolls are already expensive enough as is, so it's nice that the company makes sure its customers don't have to spend $100+ on a new doll if one is damaged. 

Let's say you're a mom of a girl whose American Girl Doll was ruined beyond belief by her younger sister. Even if you could take her to the American Girl Doll Hospital and get her spruced up, you would want to make it very clear to your younger daughter that she is never to do anything like that again. It doesn't matter how old she is: she needs to learn sooner rather than later that you cannot ruin other people's things. If she's too young to grasp these concepts, it's best not to leave her in the same room with the American Girl Doll. But if she's old enough to understand, there's no reason she shouldn't be punished for her actions.