Well, it could be a very strange or creative way of saying that the concept of an older brother figure doesn't belong in certain fictional stories about sisters. Maybe it implies that the role or character of a big brother would disrupt the narrative or theme that is centered around sisters in those stories.
Perhaps it means that in the context of sister - centered stories, a big brother character has characteristics that are too different or out of place. For example, if the sister stories are about a very close - knit sisterly bond, a big brother might bring in a different dynamic that doesn't fit well, like a more competitive or protective element that goes against the flow of the sister - only story.