I agree Fanny misses William dreadfully and at Portsmouth she might have seen more of him if he has been home since he left for sea. My question is has he been home? I am not sure her residence at MP ought to be blamed for her not seeing William. Perhaps I have missed something else in the text but the passage below suggests to me William has not been in England since he went to sea because the seven years are spoken of as one long block of time rather than several divided by previous homecomings:

This dear William would soon be amongst them. There could be no doubt of his obtaining leave of absence immediately, for he was still only a midshipman; and as his parents, from living on the spot, must already have seen him, and be seeing him perhaps daily, his direct holidays might with justice be instantly given to the sister, who had been his best correspondent through a period of seven years, and the buncle who had done most for his support and advancement… (24)

I do not remember an account of Fanny being disappointed William was home and could not visit her or visa versa. Sir Thomas let Fanny know she could invite William to MP “as soon as the squadron to which he belonged should be known to be in England” while he is gone to Antigua. Sir Thomas seems eager to get to know William in Ch. 24 so it seems unlikely he would have failed to invite him to MP had there been an opportunity.

Once, and once only, in the course of many years, had she the happiness of being with William. Of the rest she saw nothing: nobody seemed to think of her ever going amongst them again, even for a visit, nobody at home seemed to want her; but William determining, soon after her removal, to be a sailor, was invited to spend a week with his sister in Northamptonshire before he went to sea. (2)

I agree Fanny lost her sense of place and usefulness by leaving her ‘brothers and sisters among whom she had always been important as playfellow, instructress, and nurse’ (2) but it appears the only person to whom she was truly important was William. Aside from him, Fanny’s parents and siblings do not care if they ever see her again—she seems to have been as easily dismissed from their world as Lady Bertram would have ‘contented herself with merely giving up her sister, and thinking no more of the matter’ (1).