Heyy Arun....

The only age criteria that needs to be fulfilled is that the candidate should be 17 years old as on December 31, 2019. Candidates born on or after January 2, 2003 are not eligible to apply. So according to this, Yes she can ! 

For more detailed information regarding AIIMS and eligibility criteria visit below link : 

https://medicine.careers360.com/articles/aiims-mbbs

Dear Arun, as per AIIMS MBBS Age criteria, candidates must have attained an age of 17 years as on December 31, 2019. Also, aspirants born on or after January 02, 2003 will not be eligible to apply for AIIMS MBBS 2019. It doesn't mention anything about upper age limit. I would suggest your sister to fill in her nasic registration form. On January 22nd, the authorities will notify if she is eligible or not. Based on that she can fill in the final registration form.