As far as my knowledge goes, nominations made by an employee in favor of a wife, son(s), unmarried daughter(s), father, and mother are unchallengeable. If made in favor of others not mentioned above (direct dependent family members), it may be challenged, but I am not sure about it.
Treatment in Case of Absence of Nomination
Regarding the treatment in case of the absence of a "nomination," there is a provision to accept the "wife" as the legal heir. If this is contested by dependent family members, then a "Succession Certificate" is required to decide the matter, or it is referred to the Controlling Authority under the relevant Act along with a DD/Cheque. The Controlling Authority will determine the percentage of entitlement and disburse the amount accordingly.
If there is more to add, please do so, and it will be appreciated.
Regards,
AK Jain