I gonna be honest, back in the main site I already express my "problems" with that idea, namely, that Bay have two secret lives that can't coexist, her past as Brad to most of her family and friends, or her true indentity to her mother, after all, Heather is an important part of Bay's and Brady's lives, so if Mary is going to be a part of their lives now, they can't hide Heather from her. But the problem to introduce Mary to the family is that, the story that everyone knows is that Bay and Heather are sisters, both kidnaped at birth and growing separately, while Mary's story is that she only have two kids, one being Brad the clone and the other the lost "Clare" who she thinks is Bay. Tossing aside the option of telling everyone the whole true, the "easy" way for this to work, is telling the true to Mery, make it seem that Brad is another lost brother that neither Heather or Bay knew about until the wedding and let Mary help her kept her past secret.
Again, CD already said that he wants to leave this part to us and our head canons, in my case, my own head canon would be that at the wedding Heather only wanted for Bay and Mary to reunite in peace without all the drama and complications of cloning and TG guns, and after the wedding they all would need to have a talk about the true, besides, even with her memories a little mixed, I don't think Bay would like to lie to Mary about her, Heather and Brad. But again, this is my head canon, and everyone is free to come up with their own ideas.