Premarital agreement requires wife to sign waiver of claim to ERISA retirement benefits

I am not admitted in Illinois.  The pre-marital agreement is a contract between your Father and his Wife.  The Illinois Uniform Premarital Agreement Act provides legal guidelines for people who wish to make agreements before marriage regarding ownership, management and control of property; property disposition on separation, divorce and death, spousal support, wills and life insurance benefits.

Under section 7, Enforcement, it states as follows:

(a) A premarital agreement is not enforceable if the party against whom enforcement is sought proves that: 
   (1) that party did not execute the agreement voluntarily; or
    (2) the agreement was unconscionable when it was executed and, before execution of the agreement, that party:
    (i) was not provided a fair and reasonable disclosure of the property or financial obligations of the other party;
    (ii) did not voluntarily and expressly waive, in writing, any right to disclosure of the property or financial obligations of the other party beyond the disclosure provided; and
    (iii) did not have, or reasonably could not have had, an adequate knowledge of the property or financial obligations of the other party.
    (b) If a provision of a premarital agreement modifies or eliminates spousal support and that modification or elimination causes one party to the agreement undue hardship in light of circumstances not reasonably foreseeable at the time of the execution of the agreement, a court, notwithstanding the terms of the agreement, may require the other party to provide support to the extent necessary to avoid such hardship.
    (c) An issue of unconscionability of a premarital agreement shall be decided by the court as a matter of law.

Generally contracts that are enforceable by a living party are enforceable after their death by their estate, barring here, the above.  So if the agreement was valid and your Father could have forced his wife to execute the waiver prior to his death then I believe the estate can force her now.  But she can always seek to challenge the agreement (I am not saying that she will win but she can try).  You should seek legal help in all this from an attorney in your area familiar with these issues.  Good luck.