Not at all sure that you would be interested in theories, but I'll toss it out anyway. You can always ignore this.

I personally like Heinz Kohut's ideas of selfobject transferences. Sounds to me like the "aunt" is similar to what I understand is the alter ego selfobject. And most humans need, or needed those as they were growing up, something like that. Sometimes parents can do that as well as being an awesome mom, but I don't see that they have to be the same.

When people lived together in tribes and villages, I expect that that kids and adults or older siblings frequently had those kinds of relationships. But the opportunities for them these days in "first world" countries are often rare.