...of Edmund wearing his rose-colored glasses. He does say:

"...when you are her only companion, you must be important to her.”

and

"...living with your aunt, you will necessarily be brought forward as you ought to be. Here there are too many whom you can hide behind; but with her you will be forced to speak for yourself.”

So Edmund, IMO, thinks Aunt Norris is suddenly going to elevate Fanny to the status of valued companion. It's actually ironic that he thinks this of her, because she--as we know--doesn't actually want Fanny at all. But I think Edmund is a little too disposed to see the good side of people so he imagines that Mrs. Norris is inviting Fanny--though she isn't inviting her at all--to become a greater and more valued part of her life.