Marianne and her mother did not get to know Edward as soon as Elinor did, but Elinor's relationship with him was not a full six months long. From the time their father died until they left Norland was six months. Elinor and Edward were not developing feelings for each other during the whole of that time.

Edward did not arrive the moment their father died in February. Fanny and John arrived soon after, and once they had set up their establishment, Edward came. 'Soon after his sister's establishment at Norland' could be into the month of March. Elinor did not start getting close to him the moment that he arrived. It is out of character for both of them, ("his manners required intimacy to make them pleasing.") that they would be as heavily 'into' each other as Willoughby and Marianne were. They were acquainted, but they certainly, even to themselves, would not have thought that there was a 'growing attachment' between them the day that he did get there. "When his natural shyness was overcome, his behaviour gave every indication of an open affectionate heart."---how long would this take, to overcome natural shyness to the point where there would be a 'growing attachment' between Edward and Elinor? I can't think this would happen within a day or two.

He was also not there all the time (the greatest part of his time is not every single day, in my understanding) and did not necessarily start staying at Norland on the day that they met him. He was 'introduced to their acquaintance' after John and Fanny got themselves established, but at what point did he start actually staying at Norland? To me it sounds like he came and met everyone, then not long after started to stay there a lot of the time.

So, when I said 'something like Easter until the end of August, I was conservatively estimating March until the end of August (when they left--very end of August-very early September, considering a few days of travel time).

Also, Fanny did not notice anything going on until the summer.

From Ch. 4

Nay, the longer [Edward and Elinor] were together, the more doubtful seemed the nature of his regard; and sometimes, for a few painful minutes, she believed it to be no more than friendship.

But, whatever might really be its limits, it was enough, when perceived by his sister, to make her uneasy; and at the same time (which was still more common), to make her uncivil. She took the first opportunity of affronting her mother-in-law on the occasion, talking to her so expressively of her brother's great expectations, of Mrs. Ferrars's resolution that both her sons should marry well, and of the danger attending any young woman who attempted to draw him in ; that Mrs. Dashwood could neither pretend to be unconscious, nor endeavour to be calm. She gave her an answer which marked her contempt, and instantly left the room, resolving that, whatever might be the inconvenience or expense of so sudden a removal, her beloved Elinor should not be exposed another week to such insinuations.

In this state of her spirits, a letter was delivered to her from the post...

To me, that doesn't sound as though Edward and Elinor could have noticably been developing a relationship for six months before Fanny noticed it, warned Mrs. Dashwood, and Mrs. Dashwood make plans to leave ASAP.