Ask your sister to give nutrition and mothers milk has all the nutrition for babies good growing in case of not able to process then go for a formula milk which contains HMO and prebiotics in it which is next to mothers milk and helps baby in improving immunity and easy for digestion

Continue feeding that's the best way. Also add prebiotics rich food too

Continue breastfeeding, once the baby is around 8months and has started having other foods add yogurt , turmeric, orange , eggs, green leafy vegetables to the diet

its to early to comment on babies immunity right now. She need to continue feeding and secondly she need start complementary feeding at the