My sister age 15 is very tired all the time, she sleeps enough but is still very tired. i'm wondering what's wrong with her.

Tired all the time: This can be multiple causes: anemia, depression, sleep apnea, infections, boredom, stress, or other illnesses. My suggestion is that she should see her doctor for the causes of her tiredness.

See MD: Possibly anemic. See the md and she might benefit from blood work to check her chemistry. At her age it may be anemia or a hormonal imbalance. She should see the family doctor for evaluation. You are a good brother to look out for her !