Welcome and thanks for posting your query to Healthcare Magic.
After reading your query it appears that your sister is suffering from the problem of "Trichotillomania".
It is the compulsive urge to pull out hair from the areas from where the hair are accessible to pull out. It may be triggered by depression
or because of psychological effect of low self-esteem and negative attention from the family member and surrounding people can be the triggering cause.
As I said it is basically a psychological condition so it is better to meet a psychiatrist
at the earliest for the best of treatment and counseling for the problem. It is very well treatable condition and not at all serious. Along with this whole your family should support the girl emotionally and psychologically to come out the problem.
Hope these informations will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)