"Warning Spoiler" Relationship

That's true in the legal sense. However, Samantha becomes the ward of Cornelia and Gardner and is like a daughter to them. The aunt and uncle mean that metaphorically the three adopted girls would be Samantha's sisters since they would serve as the parents of all four children, even though there's no biological basis. In essence Cornelia and Gardner adopt Samantha too, but never literally go through the process so that Samantha can keep her parents' surname.