If the deed in NC is as joint tenants with rights of survivorship then the property would pass in full t
o your mother. The deed must specifically state this for the property to pass to your mother.
If the deed just lists them as co-owners without further language then they own the property as tenants in common. This does mean that the named heir in the step daughter's Will receives her 1/2 interest. This is nothing inappropriate or illegal about this unless the step daughter was coerced into making the Will or was incompetent at the time.
Your mother and this person will own the property as tenants in common, 1/2 interest each. Either person can force a court sale or division of the property on the ground if they cannot agree on how to handle the property.
I cannot provide you with legal advise. I have provided you with information about the law related to your question. My answer, and any information that you find online, should not take the place of having a consultation with a lawyer in your area to advise you regarding your specific issues.
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