To some extent, it depends on the terms upon which she was hired and the nature of your business. Employees are entitled to reasonable accommodation based on religion. If the job description was specifically to cover Sunday shifts, and the employee waited until after hire to explain her religious needs, it is possible the employee is not seeking a reasonable accommodation. However, if the requirement is simply that all employees work weekends, you may want to consider trying to accommodate her by offering her Saturdays instead of Sundays.
ASG Law offers legal advice for employers.