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IF your sister-in-law titled the property in her mother's name, then her mother owns the land outright and your sister (nor anyone else) can challenge what she does with the land or dispute what she does with the property. That said, IF the mother was not of sound and disposing mind when she made the will, then it could be challenged based on her being incompetent and incapable of making a valid legal document.
If the mother was incompetent or not of sound mind when the will was signed, then the family could petition the court to set aside the will based on fraud, duress, etc.