Edmund writes two. Austen assumed her readers shared with her. We might note that Sir Thomas is the only one in all this mess who can get out of himself, who has apparently read through and into Fanny's letters to garner how much Susan has come to mean to Fanny, and the lifestyle Susan will have to endure much more bitterly once she has had Fanny with her for a bit. Susan is of course ecstatic, and I am reminded of Auden's poem about how dogs go on with their complacent doggie life in the midst of great calamities all the people around them seem to be experiencing. Susan at least gets something out of it.
Love is blind.