Be there for her, and encourage her to get help. Offer to help out with anything you can if she's having trouble with fatigue or low motivation. Try to make yourself available by phone 24/7 so that if she's feeling suicidal or something she can call you. Offer to go with her to talk to the school counselor or even her primary care provider about seeing a psychologist and getting a psych eval.
Well depending on if you are still school age, you can talk to one of the schools counselors, or teachers or someone. If you look up depression online, you can also probably find a telephone number to call about this.