Listening to the interview, it sounds to me like she means that "you can only be so mean in a children's movie". I'd lean towards her being Old Mrs LeFay.

She's already played a political maneuverer in "Andor", so I don't think she'd be type-cast this quickly into another political leader as Jadis' sister.

I guess it is possible she would be a mean step mother / foster mother / Orphanage administrator to Polly (in contrast to Digory's dying mother who is more loving), but I feel that this subplot would almost certainly distract from the main story and does not really add much to it.

Although Denise Gough is an established actress, she doesn't quite feel on the same level of acclaimed stardom as Daniel Craig or Carey Mulligan, therefore i'm imagining she'll be someone relatively minor in the context of the movie.

Aunt Letty seems the most plausible choice of the minor named book roles, since she's of a similar appearance and age (45) to Carey Mulligan (40), therefore i could definitely buy them as sisters. Also, having her as the elder mean sister to counter-balance Mabel as the kind younger sister, seems like an interesting dynamic to explore for the film.

Other less interesting options include having her as "Mrs Potts" to Tom Bonington's "Mr Potts" (a couple of nosy neighbours perhaps?) or maybe she's just one of Digory's school teachers, etc. or some other functional sort of role.

I can also somewhat buy the idea of her as Polly's mother (as long as Polly's not an orphan per the audition script) given that, as others have pointed out, she also played Beatrice Campbell's mother in 'The Stolen Girl',

I don't see her as a sister to Emma Mackey (28/29) given the age difference, therefore i'd rule out Jadis' sister.

I'd also rule out Mrs Lefay. Firstly because she's neither mean, nor relevant enough to the story to require being mean, nor in it long enough to exhibit being mean. But also because, i think that if you are going to feature her at all, as such a mysterious and enigmatic character, but one who potentially has a such a huge role mythology wise, then i think you need to go with a big name cameo there.... As many have mentioned before, Meryl Streep would be an oddly perfect choice in that regard, but that's a whole other discussion.

Posted by: @icarus

I'd also rule out Mrs Lefay. Firstly because she's neither mean, nor relevant enough to the story to require being mean, nor in it long enough to exhibit being mean. But also because, i think that if you are going to feature her at all, as such a mysterious and enigmatic character, but one who potentially has a such a huge role mythology wise, then i think you need to go with a big name cameo there.... As many have mentioned before, Meryl Streep would be an oddly perfect choice in that regard, but that's a whole other discussion.

I agree that Meryl Streep would be a better choice for this role (in addition to what you said, 40 years old is not that elderly - "Old Mrs Lefay" sounds like she has some years behind her).

She did sound mean-spirited to me, the description of her photo sounds like it is a picture of a 'grouchy granny':

Digory looked and saw a faded photograph: it showed the face of an old woman in a bonnet. And he could now remember that he had once seen a photo of the same face in an old drawer, at home, in the country. He had asked his Mother who it was and Mother had not seemed to want to talk about the subject much. It was not at all a nice face, Digory thought, though of course with those early photographs one could never really tell.

I guess this description could mean that she was not particularly attractive, I took it that she did not look like the nicest of person.
Likewise, Uncle Andrew's further description of her makes her sound, at the very least, snobbish.  At worst, it puts her in a similar category as Uncle Andrew, where he looks down on people as being inferior to himself and feels he can break the rules in his pursuit of his own megalomaniac dreams:

“They let old Mrs. Lefay out before she died and I was one of the very few people whom she would allow to see her in her last illness. She had got to dislike ordinary, ignorant people, you understand. I do myself.

As such, I had always thought she was an aggressive, elderly woman. If they did feature her, I think it is possible that she could be a mean character.

I don't think Aunt Letty is likely for a "mean" role — she's a tough old maiden aunt, but there's no evidence that she's "mean" in either an unkind or a stingy way. She's taken her much younger sister in, along with Digory the 12-year-old nephew, at the time when Mabel is critically ill and not expected to recover, and there's no sign that she does that begrudgingly. She's exasperated with Andrew and his strange ways and his constant wasting of her money, but she puts up with him nevertheless — and we hear enough in her "fruit from the land of youth" comment (I'm away from home and don't have the book to quote if from directly) to tell that she's also grieving over her little sister's impending death.

Mrs Lefay would be an unorthodox choice, but not impossible. I also have the impression from what Uncle Andrew says about her, that she wasn't a very nice person. (Not least from the fact that he liked her.) Canonically, we only hear about her in what amounts to a brief flashback in Andrew's words, but it's clear enough already that this adaptation isn't going to go according to canon. So it wouldn't surprise me if Andrew's "fairy godmother" appears in a flashback scene that perhaps shows more of her character than the book reveals, or else she may even feature in the direct story line somehow (in Digory's present day, not in Andrew's past). 

Jadis's sister is sort of a possibility, but if we do see the flashback to the final confrontation between her and Jadis, I'm hoping they'll keep it brief. I know a lot of fans would love to see an extended exploration of Charn and its downfall, but there won't be time for that in a movie (probably 2 - 2.5 hours at most) if they're going to fit in all of the stuff that's really important to the plot!

Or Gough might be a totally new character who doesn't exist in the book. Again, with the unorthodox approach that Gerwig and co. are obviously taking to this story, pretty much anything is possible. I'm just trying to keep an open mind and reserve any judgments until I've actually seen the thing.