Jane remarked 'Marriage is a great improver-' in her Nov. 1808 letter to Cass. (Letter 61). Early December (Letter 62) she commented 'I am very much obliged to Mrs Knight for such proof of the interest she takes in me-& she may depend upon it, that I will marry Mr Papillion, whatever may be his reluctanceor my own'. Apparently, Mr Papillion was the bachelor rector of Chawton, considered by Mrs Knight as a suitable husband for Jane.
I think Jane teases Cassandra about receiving some proposal from Sir B____ as well in lat Dec. 1808 ( Letter 63)
I am somewhat reminded of Mrs Jennings (S&S) who, after marrying off her two daughters, is 'quick in the discovery of attachements' (S&S. Ch. 8)*
In her thirites, the marriage of Sir B_____ was a possible reminder to Jane of choices she and Cassandra had made, though not nececessarily regretted, maybe sent her into teasing moods on matches people suggested for herself and Cassandra-and idely, thinking up her own idea of a marriage for Cass.

* Mr Papillion owned a first edition copy of Sense and Sensibility. D. Le Faye. Jane Austen's Letters Biographical Index. p. 561.