I think, as you say, it's be because they're now related.

It's interesting in Ch. 17, Edward and Marianne are addressing eachother in first names, but at one point he says "You, Miss Dashwood" to Elinor.
I couldn't find where Elinor called him by name at all in that conversation, but in Ch. 18...
"I suspect," said Elinor, "that to avoid one kind of affectation, Edward here falls into another.
She's not quite addressing him directly here. Somehow I think if she was talking to him directly she would have said, Mr. Ferrars.