I presume your sister is otherwise healthy during day time. Ask her to avoid taking liquids after 7-8 pm. avoid tea/coffee. She must urinate before going to bed. Wake her up at 2-3 am to pass urine. Alarm clock might help. if this doesn't help, consult a doctor who might give her a drug like imipramine
. There are bed wetting
alarms in western countries but they are expensive and uncomfortable.