It depends upon what the will says but certainly with stepmother in control, things could go against you. If you feel comfortable, sit down with your father and discuss your concerns. There are many estate planning attorneys that can help put your fathers wishes in writing and try and help ensure fairness when he is gone.
Your concern is legitimate.
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If what you're trying to ask is what happens when your father dies, if the assets are all in your step-mother's name alone, or in joint tenancy with your father, then there will be no inheritance, as he doesn't own the assets. Step children only inherit from the step parent if the will leaves them something, or if they have been legally adopted by the step parent.
It depends on what your father has done. Any assets acquired before marriage are separate property unless transmuted to community property. Without a Will or Trust your stepmom gets 100% of community property. 1/3 of separate property. Obviously the Will will control if there is a Will, but only to property subject to probate.
If your dad has put all assets solely in your step-mother's name there could be issues. Same if he transferred title to joint tenancy or community property with rights of survivorship. Under either method title would pass by operation of law outside of probate.
If your dad has capacity, you really should recommend that he see a competent estate planning attorney.
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