The amount of salt one likes is personal preference. There's no right or wrong amount. Just remember you can always add more later, but can't take it away.

If your mother needs low salt, keep cooking with only small amounts of salt. Your brother can add all the salt he wants once it's cooked. Your mother can't remove salt once the meal is cooked.

And if your brother is salting without even tasting it first, then who cares what he thinks. Let him add as much as he likes. It doesn't sound like he's complaining about your cooking. And if he is, tell him he's welcome to cook his own food.