Holding a blue badge doesn't automatically mean you need a welfare seat. Thomson don't have any dedicated welfare seats. Even when they did they were just ordinary seats. If mobility is a problem you can't occupy exit seats which have extra legroom. If you book special assistance they will book what they consider to be the most suitable seats for you at the same time if they deem it necessary. Also there are certain medical conditions that do require welfare seats even though they don't need special assistance. If your mum and aunt want seats in a specific area on the plane they can pay to prebook seats and then they can choose their own.