Hello,
Welcome to Health Care Magic.
I understand your concern.
I will discuss about therapeutic issues of insomnia in detail.
There are few non specific measures to get sound sleep without medications. These are as below:-
1.     Keep the bedroom cool, dark and quiet.
2.     Wear light comfortable clothing.
3.     Maintain regular hours of bedtime and arising.
4.     Maintain a regular exercise schedule.
5.     If you are preoccupied or worried about something at bedtime, write it down and deal with it in the morning.
6.     Have a light meal before bedtime.
7.     Do not take much water in night as it may cause frequent urination and disturbed sleep.
8.     Reduce coffee/tea intake.
9.     Avoid alcohol/cigarette.
10.     Hear soothing music at evening.
11.     Do not watch TV at evening.
12.     Do not do activating exercise at evening.

Although Melatonin is effective in inducing sleep, you should consult a physician/psychiatrist rather than self-medicating.
For your knowledge I must say that Studies have found this medicine helpful in patients with difficulty falling asleep. Administration of melatonin 5 mg 3 to 4 hours before an imposed sleep period over a 4-week period decreased the time to sleep onset without affecting other sleep parameters, such as total duration or sleep architecture.
But we should also keep this in mind that insomnia is a manifestation of several other mental illnesses including depression, anxiety, psychosis and mania. So it is always better to get your sister evaluated by a psychiatrist before starting a therapy.
Secondly Melatonin has its own interaction with other medicines. So better consult a doctor.
I wish you all the best.