How did they know about 3 years they weren't going to have a child? what if they had? What would have happened to Frank??

This echoes Austen's own brother Edward's adoption by wealthy relatives, he took their name when they died and he inheirited.

There may be echoes of Fanny Price too, she's "adopted" by the Bertram's in Mansfield Park.

This indicates that the practise was likely widespread

Sign of a people pleaser! Another thing Anne and Fanny Price have in common, they want to be useful

Another link to Fanny Price who also enjoys seeing the seasons pass in the country

A similar sentiment is expressed by Fanny Price in Mansfield Park: "her wishes were overthrown ... she was so totally unused to have her pleasure consulted, or to have anything take place at all in the way she could desire" (chapter 28 MP)

We are hearing the echo of Sir Walter and Elizabeth's opinions/words. This is a strange introduction for the main character, she is ignored and secondary. Chapter 1 focuses on Sir Walter and then the family context, Chapters 2 and 3 are a group setting (and people finally speak). A first time reader may not identify Anne as the main character till chapter 4 when the text pivots to focus on her. In chapter 1 we hear of Elizabeth's disappointment with Mr Elliot but the history with Wentworth is hidden till Anne is alone. Modern texts tend to have more active, vibrant main characters (like Lizzy Bennet) who have agency and push the story forward through their choices and actions. Fanny Price in Mansfield Park is another good example of the sort of main character modern readers struggle with.

Reminiscent of Henry Crawford's desire to make Fanny Price fall in love with him in Mansfield Park.

  This chapter establishes familiar character dynamics that might elucidate the trajectory of the personas Austen presents in this unfinished text. The chapter begins with the introduction of Miss Esther Denham and Sir Edward Denham, a scheming sibling pair reminiscent of Mansfield Park’s The Crawfords and Northanger Abbey’s The Thorpes. Austen explicitly establishes the bald aim of the two to obtain wealth and status from advantageous matrimony, a characteristic that similarly mirrors the Crawfords and Thorpes. Sir Edward, in particular, resembles Austen’s past villainous men; throughout the Austen canon, coxcomb-esque behaviors are the cardinal sins of bachelors. Indeed, Willoughby, Wickham, Henry Crawford, Mr. Elton, Thorpe, and Mr. Elliot all receive biting characterizations by Austen, and thus, given the fates of these men in their respective novels, we can predict that Sir Edward is not the male love interest of this story. 
 Sir Edward’s dynamic with, and apparent longing for the affection of, Clara Brereton, additionally reverberate into the Austen canon in a meaningful way. Other Austen works present relationships between gentried men and pseudo-adopted young women; notably, Emma features Jane Fairfax and Frank Churchill’s secret engagement and Mansfield Park depict Henry Crawford’s arguably predatory pursuit of Fanny Price. These relationship both demonstrate wealth and class incongruities as interpersonal complications. Further, these dynamics are also characterized by the ignorance of other characters to the details of the relationship. Therefore, we cannot know from this unfinished account of Charlotte’s observations if Clara Brereton is a Fanny Price or a Jane Fairfax; we cannot fully know if the behaviors and dispassion Charlotte Heywood witnesses are evidence of a painful resistance to unwanted advances or red herrings to disguise an intimacy. Since speculation is the nature of this activity, however, it is notable that in both Mansfield Park and Emma, outside perceptions of the aforementioned relationships were incorrect. Therefore, paradoxically, Charlotte’s perception of Clara’s distaste for Sir Edward might in fact evince a returned affection and eventual marriage between the two.