Hi Your sister is taking meprate either to have withdrawal bleeding or  to regularise menses  .usually given to patients who are having abnormal mensus. Is she having any problems with her menses .Hormanal issues are the main cause of abnormalities in this age gp.so unless we know the reason its difficult to comment  why she is not getting periods. kindly discuss with your gynaecologist. Hope i could clear your doubts.