In a case where Elinor knew and loved the recipient less, she tells Mrs. Jennings only what she felt Lucy would wish known, although she knew many details that would please Mrs. Jennings, and increase her opinion of Edward (whom Elinor would want to be highly esteemed by all who knew him).

But it was a different time, and honour held Elinor to different mores than I am used to. Col. Brandon asked Elinor to tell Marianne, not anyone else, and the details are VERY personal. I do not know how that would play out, propriety-wise (although, I am very sure that Elinor's behavior would be beyond reproach).