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?

If you have just given birth, you don't have to worry as your milk supply has likely yet to kick in. Instead, what you are producing is colostrum which is a highly nutritious liquid for your newborn. Milk will only start coming in around Day 2 or 3. Get the lactation consultant to hand express or teach you how to latch your baby. My lactation consultant hand expressed 3ml of colostrum in a syringe for my baby on the first day. Babies also have reserves in their body, so don't worry if you don't see milk yet. I would not advise resorting to using donated breastmilk so quickly. In addition, most hospitals do not allow the use of donated breast milk due to hygiene purposes.