your sister needs to get test for serum levels of prolactin and oxytocin, the two major hormones involved in milk production and delivery.to find out the cause for decreased milk production. Breast milk is the first and best nutrition for an infant which promotes fast and healthy growth of the baby. Make sure your niece is not running fever,or has some breathing problem or has cough and cold. A very important reason why infants turn down breast feed is because of too little milk or none at all.or they have been fed too much lactogen.which is not healthy. Be patient with the baby,relax and control anxiety,do breast massaging regularly to improve breast function and i suggest you to book appointment with a obgyn to run some tests to find out your sisters low breast milk production cause. hope that answers the question