I think that you may me right on this point. Lets think about it. Ranveer might be the one that is blackmailing her. The pieces of this puzzle seem to be fit in this situation.. Maybe Ranveer's motive was always to obtain the property. Bani received the white sari, and the address of the shipper was listed as the Walia Mansion. She received the threat against Jai's life, and she has to act this way in order to save him. I don't think that Bani has turned bad although it might seem like it. We know from the beginning that she loves him, and we know at the scene where the raaz was being revealed she loves him despite what happened. She would be ready to die for this man, because she wanted to die with him and not be alone. Ranveer could have access to anywhere in the house, therefore all these letters can be placed in files and folders with only him knowing.
I said earlier that this could very well be Tony. And for all we know they are in this together (due to the white gloves).