In many descriptions of the film, Birgit is described as Katja's best friend. In the film she is called "aunt" ("Tante") by Katja's son. Please add further information here if you can.
Not sure of the relationship, but I would note that many people often refer to friends as "aunt" or "uncle" when addressing their children, probably to create familiarity, but also to keep a certain distance and respect (i.e., prevent the child from addressing the adult by their first name).
