"She is Mr. John's aunt."
It's a bit weird, but it's not wrong to use Mr Firstname. It's kinda old fashioned or slightly childish, but some people would do it, for example, referring to a son of a family they greatly respected. The father would be Mr Surname and the son would be Mr Firstname.
Is 彼女 kind of odd to use here? like i understand why they're calling for it, but my intuition says that it sounds like it's referring to a younger person (or coming from a very old speaker).
(i grew up hearing native Japanese spoken, but am not a native or like-native speaker)