]Truly I don't understand how poor Elinor is accused of lack of compassion if she's so concerned about her sister's welfare. Is there something I'm missing with the customs of the time regarding engagements that you just shouldn't ask people or is Mrs. Dashwood just being totally unreasonable, because frankly I think she is. Much of Marianne's sadness might be averted if her mother probed a bit.

From Jane Austen's World:

"There was much behavior prohibited to a young couple before they were engaged. This included using Christian names, unless connected by family, driving in carriages, correspondence; exchanging gifts and any kind of intimate touching. If any of these were observed to take place, then the automatic assumption was that the couple were engaged.
Marianne and Willoughby are guilty of all five kinds of transgression."

I suppose Mrs. Dashwood has good reason to assume they are engaged and if I remember correctly, Elinor feels they are in all likelihood engaged because of their behavior also. However, as time continues to elaspe with no announcement of such an engagement Elinor being a perceptive and rational person and never certain anyway begins to feel uneasy suspecting that there is at least the possiblity that they are not engaged. IMHO she is concerned that if this should turn out to be the case, Marianne's reputation and honor is at risk. Mrs. Dashwood refuses to consider any of Elinor's concerns and I have to agree with you that she is being very unfair to Elinor and lacking in her responsibility as a mother. I think that the sisters in, all probability, have basically had an open and sharing relationship even when in disagreement until recently. Because Marianne is now keeping whatever the relationship is to herself rather than sharing, Elinor feels that she is being purposefully excluded and that to approach Marianne would offend her whereas, their mother has the right to ask and counsel her daughter.