I know it was not explicitly said but when Raghu said that her trusted lawyers can cheat her for money it was easy to understand.
If we remember that in the initial phase of Diya Ratan fake romance, Kaki ma sa tried to figure out their motive and even became desperate in trying to kill her.
But later, someone like her is expected to get her poise back. Her planning to discredit Diya might have started much earlier (getting her sketch done instead of Priya) but initially it was mainly to save Priya.
But this getting real papers made and attack on Ratan in engagement was done after the property transfer announcement.
It seems one of the assistant lawyer had leaked the matter of fake papers to her and after knowing that it was an educated guess that Diya and Ratan are faking it.