it's one thing to decide that she has an aunt without an uncle or a cousin. It's another to realize that's going to blow out of the water a scene that incites the whole story, when her mother's attempt to protect her from feeling left out collides with reality that she's not being left out.
Perhaps I could write the story without it. I know I am going to have to change other scenes, where the neighbor girl (who actually was a character, just not appearing that early) would not act as the cousin would have. Less friendly and more reserved, not to say icy. (She has reasons.)
But moving her forward would also get all three main characters on stage quicker.