Ask yourself sister to stay well hydrated and include protein in her diet. Mutton soup helps big time to boos the milk supply. Also ask her to feed the baby atleast once in a hour it will boost the supply. If still she isn't able to get good supply then it is best to look for formula which is close to breast milk and contains HMOs abd prebiotics. My sister has issues due to low supply and our doctor suggested us to start with Aptamil Gold stage 1 for my 2 months old nephew. I would highly suggest you to check with your pediatrician first

Hi Diana, she can try having shatavri powder with milk to increase the breast milk supply and also ask her to have methi seeds. Aside from this to ensure baby gets enough milk and nutrition you try giving the child formula milk. Pick something that meets the nutritional demands just like breast milk. I have heard a lot about Aptamil Gold being prebiotics rich and also is closest to mother's milk. It helps with gut health since prebiotics acts as food for the good bacteria there by improving the digestion capacity and boosts immunity as well.