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, please do not act upon said opinions without consulting with a suitably qualified professional.