- Ella tiene veintidós años.

First of all, let's analyze the question.
Question:
- ¿Cuántos años tiene tu hermana mayor?
From this question we can extract two pieces of information.
- It is ""hermana" (sister) that is, it is a girl, if we want to substitute with a tonic pronoun it must agree with GENDER and NUMBER. In this case, it would be replaced by "ella" (she).
- Assuming that the girl answering the question is under twenty-two, then she is the one with the highest number "veintidós" (twenty-two).
Note: If the girl was less than 7 years old, then that would be another option, but it would be very rare for a girl 6 years old or less to answer that question.