If your mother qualifies as your dependent, then you can claim her. But she would have to file her return and indicate that someone else can claim her as a dependent.
To find out if you do qualify to claim here go through this IRS Interactive Tax Assistant - Who Can I Claim as a Dependent?
If she filed her return and checked that she is not a dependent, then you claim her as a dependent and efile, the efile will be rejected. She would need to amend her return and check the box that someone else can claim her as a dependent. You would need to print and mail your income tax return and include a letter to the IRS about claiming your mother as a dependent after she already filed.
Your mother would be a qualifying relative if she meets the qualifications.
