I agree that it's quite likely that Emma and Mr. Knightley will invite the Martins to dinner at Donwell. I think, though, that it will be like it was probably done at the John Knightleys, with just their family. The Martins will be welcomed at informal dinners with just Emma and George and will likely be just once in a while.

On the other hand Bridget is also right when she says that certain ties have to broken and Harriet and Robert are of the second set, if that, so will not be invited to dinners where others are also invited. Even if they themselves did not have a problem with it, it would be unfair to impose a breach of the codes on others. It's kind of like with Miss Bates who is a gentleman's daughter, Mrs. Bates who is the widow of a gentleman and Jane, also the daughter of a gentleman who are higher up the scale than the Martins, as well as life-long friends who are not invited to dinner but only the after-dinner gatherings. At least, that's how I see it. :)