It’s understandable that your sister is going through bereavement . It includes different stages disbelief, shock , numbness and feelings of guilt, anger , tearful, preoccupied with deceased, loss of sleep and appetite. These symptoms can last for 6 to 12 month . If she is getting very distressed and symptoms are affecting her daily functioning then we treat it as acute stress reaction/ adjustment reaction.which needs short course of medication combined with counselling / therapy

Consult a psychiatrist... N this could be a grief reaction which is a normal phenomenon... Need more details if patient has any other symptoms besides grief...