The answer depends on whether your father left a will indicating his wishes -- or on where he lived and died and how he held the property.
If he left a valid will, then it rules -- and the property must go to the people or places he indicated.
But I suspect you are asking the question because there was no will. In such cases, a deceased person's property is distributed at death according to a percentages and a hierarchy of relatedness called the laws of intestate succession. And that's where the complications set in. For example, many states provide that a surviving spouse is entitled to take all property that is owned as community property -- that is, all property a couple acquires together during the marriage, except for gifts or inheritances -- and can take a percentage of all other property, with the rest being distributed equally among the natural children.
To find out the specifics of the law that will control your situation, do an Internet search of "intestate succession" plus the name of the state in which your father died.
If it is unclear to you how the law may apply in your case or if you remain worried that your stepmother will not abide by the law, it might be worth your time and money to hire an experienced estate planning attorney for a little help. The more information you can provide about the type of property your father owns or will own at death, the more efficient and cost-effective such a consultation will be.