Your brothers claim would go through a special track of the courts as well which would require a solicitor, so additional costs.
Have you actually had a survey carried out and proof that there is a failure in the DPM? Where is the damp located? If it's a dpm failure then it will be very obvious, low level localised damp staining to lower walls etc.
The opinions I give are simply my opinions and interpretations of what I have learnt, in numerous years as a property professional, I would not rely upon them without consulting with a paid advisor and providing them with all the salient facts BSc (Hons)