Marianne's tendencies are so well-known to Elinor, that she can see that the minor displays that Marianne can not help are truly advances towards heroism (ch. 37).

I love how much the sisters take care of each other. Too often, the leads in books and movies these days are so wrapped up in their own problems that they do not realize that they are making it worse, both for themselves, (because they are not getting the support that might be offered if they sought it in the right way) and for others around them, including some they claim to love, who might be having as bad a time.

Just re-read the passages in ch. 32 that describes Mrs. Dashwood, Elinor, and Marianne's reactions to the idea of the daughters staying longer in London: Mrs. Dashwood wants them home, but decides against her own comfort when she thinks that Marianne would suffer more if she returned. Marianne deeply regrets having no access to her mother's personal sympathy in this most trying of times, yet is glad for Elinor's sake. Elinor, thinking she can not avoid Edward completely, is consoled by thinking that at least it is the best thing for her wounded sister...