Gouri Ma is Anju's mother and Nalini is Sudha's mother. The two women are pregnant at the same time, and their husbands are cousins to each other. This makes the two soon-to-be mothers quite close. Both women go into labor at the same time as well, because both women learn that their husbands have both died. Because of all of these parallel coincidences, it shouldn't surprise the reader to learn that both Sudha and Anju are born on the same night as well.
With the above general summary, it would make sense if I said that Anju and Sudha were both born at the exact same minute too. Not the case. Anju was born first, and Nalini's labor and delivery was not going as smoothly. It was taking a lot longer than Gouri Ma's delivery had taken. To help Nalini focus and take her mind off of the pain, Gouri Ma placed baby Anju on Nalini's pregnant belly. Sudha was born soon after. Both Gouri Ma and Nalini say/believe that Anju must have called out to Sudha to come out of her mother's womb and be born.