Bimla already knew that her son was dying. She has known all along that Ganga and Jagya are at the Jaitser hospital. I think she deliberately brought him to this hospital so that she can try and request Ganga to let Mannu meet Ratan.
She probably did not know that Ganga has become a doctor. Which is why she was surprised when Ganga told her that she would treat Ratan.
Now Ganga has heard the request directly from the party involved. She is probably unsure of what to do.
On the one hand she is unable to decide whether or not to tell Mannu since she is worried about his reactions and also the effect that it will have on the family. On the other hand she has told Jagya in no uncertain terms that she will tell Mannu if Anandi tells Shivam about Nandini.
Even if she decides to tell Mannu, the condition she placed in front of Jagya could hold her back.
Anandi knows about Ganga's condition. And Ganga knows that Anandi knows.
Under these circumstances, should not Anandi tell Ganga that she has informed Shivam about Nandini. At least Ganga's decision on whether or not to tell Mannu will not be held back by Anandi. Else Anandi will once again be blamed unnecessarily that the only reason Ganga did not inform Mannu is because Ganga had no knowledge that Anandi had already informed Shivam.