Mahtab, according to NEET eligbility criteria, for a student to be eligible for NEET, he/she should have studied his/her 12th  with Physics, Chemistry, Biology and English as core subjects. Only Biology is accepted as an additional subject but English should be a core subject. So unfortunately your sister might hot be eligible for NEET. Having said that, we would still suggest you to reach out to NTA for their take on this. You can mail them at: neet@nta.ac.in