I was struck by that passage in Chapter 9 also, I suddenly saw Marriane as being much younger and I could appreciate Austen's earlier remark that Margaret had "already imbibed a good deal of Marriane's romance". As you say the scene from Chapter 12 is obviously confirmation that they have been discussing Elinor's love life. What strikes me is the greater degree of openness between Marriane and Margaret. After Willoughby's hasty departure we learn that neither Elinor nor Mrs Dashwood have had actual verbal confirmation as to whether or not Marriane and Willoughby are engaged. Elinor obviously does not feel she can openly talk about this subject to Marriane, I know Elinor is naturally more private on such matters but surely even she could tempt a cautious enquirary. Was there any kind of social etiquette that prevented her? Or could we see this as evidence that Margaret and Marriane were more alike, and therefore more likely to have private chats both about their own love lives and Elinor's.