Elinor is saying that Marianne has been spending a lot of time with their mother, sharing her grief. Actually, I think Elinor is being kind to Marianne in this conversation, calling her inattention to others not a failing of good manners but an understandable action due to being a loving daughter to their grieving mother.

We see from Chapter 1 that Mrs Dashwood & Marianne "gave themselves up wholly to their sorrow, seeking increase of wretchedness in every reflection that could afford it, and resolved against ever admitting consolation in future." I suspect that this behaviour lasted long enough that Elinor was the only one of the Dashwood ladies who bothered to spend time with Edward during the early part of his visit(s).