In a thread below this was quoted:
"Mary never wrote to Bath herself; all the toil of keeping up a slow and unsatisfactory correspondence with Elizabeth fell on Anne." (ch 12)
It made me think - Mary seems to be entirely on the outside of the Elliot family. They don't appear to visit. Sir Walter seems to have little to no interest in his grandchildren. He never mentions them. Sir Walter and Miss Elliot can't communicate much with Mary as Mary never writes.
We hear of no visits. Although Mary complains Lady R. has not been in the house above 3 times. She doesn't complain about Elizabeth or her father failing to visit her. Do they visit? She could just be resigned to her fate as distant from them and instead reliant on her new family to entertain and love her.
She seems (to me anyway)somewhat jealous of the close friendship of Louisa and Henrietta. I think she has some real reason to feel left out. But she doesn't ever express this directly. But she isn't the type to feel this directly and express it out loud.
Still it seems if Anne is overlooked, Mary seems not to exsist at all. If I were leaving my home for a long period of time, I would probably at least visit my sister before. But Elizabeth makes no such effort (that we can see).