Mary isn't treated any better by her father and Elizabeth than Anne is.

Mary does not correspond with Elizabeth, that is left to Anne. I can imagine Elizabeth being completely miffed that her youngest sister married before she did and rose in ranks to take her place. With Elizabeth having the same Elliot pride as Mary, that would explain why the two of them no longer speak.

Her father Anne likely sees infrequently -- I can imagine a day visit by Sir Walter to the Musgroves as politeness would dictate, the obligatory morning call on his daughter to see his 2 grandsons, and on away home again.

I wonder what Sir Walter's reaction to Charles' request for Mary's hand in marriage was? We know LR did not approve of him for Anne. Would she have had the same reaction for Mary? Did SW give no direct reply, neither yes or no, to Charles as he did for CW because the Musgrove family was not up to his expectations of grandeur and wealth? I have the impression that Mary would have been more headstrong than Anne, more determined to marry (especially if she harbours jealousy against her older sisters), but perhaps married for not all the right reasons...