As a U.S. citizen, your mother is considered an immediate relative. This means she can pursue adjustment of status within the United States or an immigrant visa through consular processing. PLEASE NOTE, your mother cannot enter the U.S. on her tourist visa solely to pursue adjustment of status within the United States. This would mean she has entered with immigrant intent on a nonimmigrant visa, which can lead to accusations of fraud. If you are to pursue adjustment within the U.S. for your mother, I would suggest consulting with an attorney.