Nimboli's current sufferings even if she doesn't get to know that she is Nandini are far less than what she went through under the watchful eye of the greatest mother on earth Mangala.
Only problem is that Shivam is acting out of insecurity.All he needs is an assurance from his mother that nothing would diminish her love for him and he will be fine and he will put up with the girl,
So even as an orphan she has a better life with Anandi.
Identity as Kundan's beendini and Mangala's daughter didn't do her any good.
She calls Shivam Shivam Bhaiji so it is clear that she considers him as brother.
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With Anandi even if the girl is really an orphan with the kid of life Anandi is giving her she will be able to carve her own identity. Here the child is Anandi's daughter and she is already getting everything and her knowing that she is the daughter is only matter of time
So the child as long as she is with Anandi she has a much better future and if she doesn't have an identity because she indeed is an orphan even then a much better life.
For Mangala child marriage is nothing.If Kundan pretends to have changed she would happily make the child Kundan's beendini again and will close her ears to all the screams that come out of Kundan and his Beendin's room night after night telling herself that she has given a safe future to her chhori.😛
Anandi would rather have the child hate her and refuse to accept her as a mother than see the child go through all this as a child bride of Kundan.