Dear Brother..Welcome to HCM,
The problem that you briefed about your is sister is pretty common in these days across the current generation of young folks.....The reasons could vary from nothing to everything..
Medical treatments for this problems will not be a cure on a longer run...I suggest you to take your sister to a Psychiatrist
to unearth the cause of her depression
..once we have this on our hands..we can always look at ways to help her over come the depression with some amount of counseling supplemented with a small dosages of medication...
For now as a first step, consult a good psychiatrist...
Love and suport from your family towards your sister will help in bring her back to normalcy very soon..Give her the Trust and sow the hope in her ..