Mandira challenged that she will take away Nadini's child.And she wanted Nadini's child only not Tanya's.(She said this to Karan)
I think Nadini gave her child to Mandira to save Tanya's son.Remember Meera kidnapped Tanya's son.She might gave the child to Mandira(I'm sure Manthan is not their own son),then Mandira might blackmailed Nandini to give her child otherwise she will kill Tanya's son.
So now Nandini might raising Tanya's son & Mandira raising Nadini's child.
[OR.... Nandini's Kid was kidnapped by Mandira when she was delivered,and doctors must said the baby is dead.After 5 yrs when she came out She adopted/raising Abhir(that time he must be arround 10 yrs )He must be calling her Aunt.(now that makes sense for falling love with Bhoomi.)
(Joydeep also adopted son of Sahil ...Ekta mentioned that he is going to marry Bhomi)]