I notice the servent gives the letter straight to Miss Bennet then awaits her reply. Mrs Bennet all the time exclaiming aloud for details. (Is she expecting a proposal already?) I don't think in this case she had opportunity to snatch Jane's letter! Yes, Lydia and kitty do engage in quiet correspondence easily. But as an exchange of letters between two sisters, I don't think Mrs B. imagined there was much to interest her. All of Lydia's underlinings in her letters were possibly regarded as simply like silly girlish secrets Lydia and Kitty giggled over at home. Another e.g of real lack of concern by their parents? But a letter from a gentleman. Aha! Could Mrs B. resist? ...Perhaps the Mrs Bennet of the novel would take it staight to Lizzy and demand it's contents be known to her right away?