ask your mil to speak with a lactation expert. the world health organisation says nothing bit bf for first 6 months.

All the very best Mom

Breast milk is 88% water. your baby meets all his water and solid needs through mothers milk. Water tends to make the baby feel full and may not nurse effectively, which may affect mothers milk supply. It may result in loss of weight in baby.

1st 15 min baby gets as good as water via direct feed. so no need of external water

If your baby feels thirsty then give her breastmilk...DO NOT GIVE PLAIN WATER. It may lead to water intoxification.