The CBSE standard is very high, and is far ahead of any State Board. This is one truth that should be accepted.
That said, please do note that the transition to the CBSE syllabus is not that very difficult. May be, a little bit of private coaching for your sister, in mathematics, may only be needed. The secret is to join the school immediately, and get your sister coached on the portions of the CBSE, even now, before the school re-opens.
Those in the CBSE standard of education, find it far more easy and naturally more able, to crack the IIT JEE or the NEET examinations.The standards in these competitive examinations is much higher than that of the State Board.
So, just do not hesitate. Counsel your sister and instill confidence that she can do it. She will. In fact, she can launch herself into whatever suits her best, in the plus two stage.
All the very best.