How Far Does Isabella's Letter Create Such An Important Moment Of Focus In The Novel?

I am asked to evaluate how Isabella’s letter create such an important moment of focus in the novel. The interpretation of the question is crucial for developing an appropriate answer, and I believe the question is asking how the letter is such a predominant part of the novel and why this is.
In the letter, Isabella seems to emphasis her view of Bath by repetition. She states, ‘this horrid place’ ‘this vile place’ and proceeds to say, ‘Since you went away, I have had no pleasure in it” Isabella is trying to get Catherine to console with her or even gain her sympathy. She fails to mention any primary reason of her newfound hatred of Bath other than Catherine’s absence; which sparks some suspicion of interest regarding the letter as Isabella is being far too kind.

However, it is made clear rather quickly. ‘I am quite uneasy about your dear brother, not having heard from him since he went to Oxford: and am fearful of some misunderstanding.’ Immediately we are aware of the fact that Isabella is really writing this letter for her own gain. She states that she is in love with Catherine’s brother and she believes that if Catherine were to speak with him they could be together. It is rather clear that the subject of the letter is Isabella trying to convince Catherine to write to her brother in the hope that he replies. Isabella tries to deceive Catherine into thinking she cares about her, however the real reason for contact is submerged in a mass collection of rather useless information.
The irrelevance of many parts of the letter makes it feel as if you are reading a Journal entry, certainly not a letter to her ‘dearest Catherine’. Isabella begins of on a tangent regarding Mr Tilney, but ensures she adds a sentence regarding Catherine brother shortly after. Everything in this letter has to return to the brother in some