I think framing Ronak is Robbie's plan, in 2day's episode he was most bothered about Kasturi rather than his sister's death. He also said that he is getting the divorce papers ready & no one is going to depart them anymore. So by this it's clear he is the one who is behind this game plan to trap Ronak to get back Kasturi, I think the last scene might be Robbie's dream or may be Kasturi is just pretending to find out the truth.
But i also have a doubt that Robbie shall use such a ghatiya plan i.e., muder of his own sister to get back Kasturi, may or may not be.