Very nice of you to be looking to solve your sister's problem. Just two problems: not going to college and tension do not qualify for depression. There are many adolescent and young age problems that may lead to such symptoms. A lot more information and thorough interview with your sister will make it possible for a psychiatrist to diagnose and treat her. Kindly see a professional immediately.
Your sister could be suffering from Adolescent School Refusal Disorder. It is more of an anxiety disorder than depression. Panicking, palpitations, giddiness, sweating, occur in spells. She can read the books and syllabus, her IQ may be above normal as well. Everything else could be normal except for going to college. She will respond very well to medications and Counseling or therapy. Kindly consult a psychiatrist. The treatment can be slow and tedious but may be well worth keeping in mind her education, career, confidence and life goals that are at stake.