Sorry to say you but girls are not eligible for NDA exam. Only boys can go to apply in NDA exam. NDA does provide permission for girls.

Detail for NDA exam:

Eligibility:

For Army wing of National Defence Academy
Candidates have to pass 12th Class of 10+2 pattern of School Education or equivalent examination conducted by a State Education Board or a University.

For Air Force and Naval Wings of National Defence Academy
Candidates should be 10+2 passed with physics and mathematics subjects. Candidates who are appearing in the 12th class under the 10+2 pattern of school education or equivalent can also apply.

Age limit:
Candidate’s minimum age is 16 1/2 years and maximum age 19 years for the NDA exam.

Sex:
Only unmarried male candidates are eligible.

Your sister can continue her dream to be a part of defence after graduation through various entries.

OTA as an officer through CDSE entry

IAF flying branch through SSC and AFCAT

She can be a ground officer in IAF through CET