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Sorry you are dealing with this situation. Where or not a legal claim to the title of the property can be sustained will depend on how the property was titled between the husband and the surviving spouse (your father and step-mother).
If property was titled as tenants in common then you may have a claim to your father's half of the property.
If the property was held as joint tenancy with right of survivorship, on death all the interest transfers to the surviving joint tenant and then that owner is 100% owner and may do as they wish with the property.
You will have to obtain a copy of the deed to the property and determine how the property was titled. If at anytime she was a joint tenant or became an owner in the property after his death, she may dispose of the property as she desires.
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