My mum also wanted to sleep train my baby by suggesting giving water at 2 months old. I told her straight away it’s not advisable by the doctor and shut her off. Have to be direct and firm.. and yes get your hubby to follow. Breast milk is mostly water content already, don’t need to give water otherwise they don’t have enough nutrients when the stomach is full from water.
Go to doctor and get doctor to issue the dangers of baby below 6 months drinking water. But most importantly, your husband have to stop first else your mil probably thinks: Even the dad also give, there’s no wrong.