This is a very sticky wicket; normally if a spouse is designated as beneficiary if an IRA, barring a property settlement agreement or divorce degree, that spouse has to sign a written consent to allow the beneficiary to be changed! I urge you to consult an experienced probate attorney.
There are also others rights, claims and/or allowances a surviving spouse can act on, a seasoned probate attorney in your area can advise you on all these!
IRAs are not subect to the requirements of the "Retirement Equity Act of 1984" (REA) so beneficiaries other than a spouse may be entiteled to the account. But, this also depends on state law and the IRA custodian agreement. If your father and step mom were "legally separated" at the time of his death, your chances are even better. Please take a copy of the IRA custodian agreement to an attorney who is very familiar with retirement accounts and the REA for review. This is one of the areas of the law that can be more difficult to understand. Good Luck.