Mary seems so much more sophisticated than Fanny, that I can't see them having an equal exchange... Mary can be kind with Fanny, but I could also seeing her lose interest in Fanny, as the Bertram sisters often do. If Fanny can be of use to Mary in some way, I could see the interaction rise. And again that could be in the direction of Edmund. For Fanny to seek Mary out - I don't see that, again, unless it had something to do with Edmund or kindness from Mary (which hasn't completely happened when it comes to the pony, etc.)