I get that, but I just…don’t see it. For me, the references are to answer the question the audience would have about why Christina wouldn’t be there when one of her daughters was brutally attacked, or why she’s still not around when both of her daughters move to New York after narrowly surviving the ensuing bloodbath.There's too many deliberate mentions of her for us not to meet her. She could have just been dead. Why bother bringing her up at all?
The fact she had a daughter with a notorious serial killer, and that daughter then went on to implicated in copycat murders and develop her own psychological issues, makes Mrs Carpenter a potentially really interesting character. How troubled is she herself?