Above you say that Mrs. Norris caused the breach between the sisters, resulting in Sir Thomas not being inclined to help the Prices. The way I read it, while Mrs. Norris certainly laid the groundwork, Mrs. Price's "injured and angry" reply that "bestowed such very disrespectful reflections on the pride of Sir Thomas" was what actually did the damage in Sir Thomas's mind. Perhaps if Mrs. Price had answered in a more measured way, things might have been very different. I suppose you could argue that Mrs. Norris didn't need to share the response with the Bertrams and thus caused the breach, but I imagine Mrs. Price knew her eldest sister well enough to realize that anything she wrote to Mrs. Norris would be passed along.