I think, as in many things, the rigidity of rules of mourning were pretty much an artifact of the Victorian era rather than the Georgian. Even during the Georgian Era, rules of mourning were more in view in the city and among people of fashion. Degrees of mourning and the dress expected for those degrees seemed to vary more among the gentry, particularly those for whom the cost of fabric and construction was an issue.

The only rules that you will find documented during our era are those for public mourning for a deceased royal or for a national hero. These rules, however, were primarily observed by the upper classes and were published in the various ladies' magazines of the time.

However, I think it more likely that Jane Austen's postscript to her sister referred to what clothes Cassandra would like her to send to Godmersham for her.