First tell you sister to pursue an Undergraduate Degree after 12th be it B.Sc./BBA/B.Tech/BCA/MBBS/B.Com/BMS/BA or whatever she's interested in. Preferably an Engineering, Mathematics, Statistics or Commerce degree will help her, since shell ultimately pursue MBA.

In the Final Year of her graduation ask her to appear for CAT followed by MBA, or she may have a short 1-2 year Work Experience followed by MBA in some of Best Management Institutions in India.

For more information about CAT do follow the link below

https://bschool.careers360.com/exams/cat/amp