Can I ask the nurses in the hospital to feed my newborn with my sister's breastmilk as I have very low supply? Anyone here has tried that before?

I haven't tried it but breastmilk donation is so common now. At least you know that she is your sister and she does not have any infectious diseases that may pass through the milk. Although it may be discouraged because of hygiene issues I guess you could trust her since she's your sister. http://sg.theasianparent.com/zoe-tay-milk-sharing/2/ My concern is that your sister has a much older baby than yours and the milk composition may not be the most ideal for your baby. Breastmilk from mummies with bubs less than 6 months old tend to have thicker milk to help baby with weight gain and growth. Those who have older babies tend to have more diluted milk that consists of more antibodies to boost immunity.