I think your BFF is wrong on both counts.
She should pay to see her niece's recitals. She should pay to see your kids' school musicals. She is the audience member.
I understand this: she has to travel and then suffer through the other children's "talent." And she has to be around her unpleasant brother.
Not one of these things is justification for her to demand someone else pay for her ticket to an event she wants to attend. (She must want to--she goes.)
I suppose there's an argument that says, "You invited me to come to the recital," but honestly I would have to have received a very explicit invitation, as opposed to a "it's her recital, would you like to attend? here's when it is" which implies that it's a notification, not a hosted invite.
So I believe she's wrong there. I don't think that she has received a "let us host you!" invitation to any of these events. I'm certain you didn't give her one.
Yeah, it means she's paying to see the performance of someone else's child. But these children presumably have a relationship
with her, right? They are "her nephew" and "her friends." If she doesn't care about them enough to fork over the admission fee, she should stay home.
And multiple nights? Oh no.
Even if you were going to accept her contention that the parents have to pay for the tickets for their children's fan club members, parents would pay *one* night. The night of their choosing.
I think you should call her bluff and say, "Oh, well, then we won't ask you to go to the show. You can stay home; I'm sure it'll be more fun for you."