I think she is the replacement Granny in the sense of becoming central to the Witches arc of books - she will be written about in the same books as Nanny, Agnes etc when Granny is gone.

I also think she will be the replacement Granny in terms of status within the witch 'community' - see 'The Sea and Little Fishes' (short story) for an example of the kind of thing I'm talking about. We've already seen Tiffany being shaped up by Granny for this role.