It also sounds a little spiteful - Caroline was aware that her brother liked Jane, probably suspected Jane was partial to him - and so she writes to her 'dear friend' Jane of furniture !! As well as showing her near connection to Darcy's life, she is avoiding giving Jane as much information on her brother as she can, instead focusing on Miss Darcy and on chairs and tables :) . She's clearly displaying her lack of concern for what she knows must be one of Jane's real interests.