That was me under a different name. (I change frequently. To cover up my criminal past. 😀)
It was based on nothing more than the fact that Sally mentions in an interview (but only once that I could see, and nowhere in the books) having a sister who’d done what was expected of her and married a farmer, and ‘Polly’ lives on a sheep farm in the midlands, and is presented as having known Sally in her childhood and teens, having been in contact since they lost their home, apparently knowing about the eviction and Moth’s illness, having their phone number and knowing they were on the path.
So nothing conclusive. But the sense of rivalry RW feels with Polly does slightly suggest possible sibling competition to me. If their parents always criticised Moth for being a ‘lazy’ ‘townie’ with no land, and the sister married a farmer and clearly has a stable life, it wouldn’t be surprising if Sally felt the entire family were saying ‘Told you so!’ now she finds herself living in her sister’s outbuilding.
Also, significant that she doesn’t go to her own family for financial help after she was arrested, but Tim’s? Could just be that the Walkers are richer, or suggest estrangement from hers, and explain the negative portrayal of Polly, even as she gives them somewhere to stay from October through to the following August.