I am no expert with these things, but I understand that if your brother is still in full time education, your mum will still be responsible for contributing to his upkeep. Even though he is not staying at home during term time, your father will have to maintain a home for him and any financial support your brother receives will be based on your fathers income - probably including any CSA award. What puzzles me is that it has taken 2 years for them to ask for the money - surely when your mum stopped paying, someone should have assessed whether the payments should continue or not.
Sorry if this is not much help, as I said, I am no expert, so I think that your mum will need to deal with the CSA and if needed get professional advice - can the CAB help?