I strongly recommend your sister change her study habits. If she's been working with a tutor, she might do better with a different one.
Since you're not the one studying for the test, she's a 3rd house matter. She's symbolized by Mercury, which is not in a strong position, in the sign of its detriment in the 8th house.
With an exam question, I always think that so much depends upon the person, not just the stars-- how hard is s/he studying? Is she studying the type of topics likely to be on the test.