I am surprised that Ranbir's parents haven't yet showed any concern about Durga. How could they let Ranbir and Durga stay at the Garodia house for so long? Durga was feeling sick and Ranbir pretended to the Garodias that he will have to leave work to take care of her so the Garodias insisted that they stay with them. Well, weren't Ranbir's parents shown as quite wealthy? They could've asked for a nurse to take care of her regularly. Another thing is that his parents didn't even come to meet Durga nor have they called to ask of her well-being.
If Ranbir's parents had interacted with Durga and Ranbir after marriage, his mother would've definitely been doubtful that this so called Ranbir is not really her own son. It would've been nice to see her supporting Saraswati and helping her fight for the truth.