(a) Since the sons of Nirmal were majors, did she have a right to relinquish the inheritance to her brother.
Nirmal is the legal heir of the deceased mother and it was the self acquired property of the deceased so the children of Nirmal have no rights in the property hence their permission or consent was not necessary.
(b) Did not the DLF err in transferring the plot without the RD's of the sons of Nirmal.
No absolutely not.
My mother Nirmal Malhotra, who is alive does not remember signing the Relinquishment Deed !! and is willing to state on oath and give an affidavit of not having done so.
If she want to fight for the property, she can do it by filing a partition suit now, but only she is eligible for filing a suit however she may not be able to win the case since she had released her rights very long ago.