If the works go ahead, and if your brother can accept slightly reducing the bedroom size, trying something like this would obviously be less expensive than moving and might reduce noise levels to an acceptable level?Thanks again. Yes, Moving is something he mentioned, but can cost a lot. I think it's mainly the fact that it's his bedroom one side of wall where you need it quiet and a kitchen other side that can be a noisy working area with all hard surfaces that amplify the noise. You wouldn't design a house with bedroom next to kitchen.
He's adamant he won't allow their roof to be raised which will scupper the project. It's not just the potential for noise, but their total lack of communication and consideration about the work.
How to Soundproof a wall against noisy neighbours? (soundproofingstore.co.uk)