I agree; I've wondered if Cassandra's letters to JA may have 'filled in the bits' cut out of Jane's letters- though I do not query Cassandra's right to do this.
Cassandra's letters may've been more interesting to tell us about Jane, jmo.

Jane's preserved letters may reflect the mutual interests she shared with Cassandra; nephews & nieces, family acquitances, clothes yet so far, there are no references to Jane's particular interests, such as music.