In reply to Dominicandave: Carry on returning the letters as you are ding - don't open them.
Next time they phone explain the situation again. State that they are harassing you and you would like to make a complaint and ask for the name and address of the person you should write to. Send the letter recorded delivery and keep a copy. If they don't provide details then state that you will be making a complaint to the Financial Services Ombudsman. Do this, but again keep copies of letters.
In relation to bailiffs, whilst it might be stressful if they do turn up, they should not be invited into your house, and they can't break in or force their way in. Even if you do invite them in they can't take anything that does not belong to your brother.
It might also be worth contacting your local authority to ensure brother is not still registered at your address on the electoral roll. Could also contact Experian and Equifax (credit rating agencies) to see if brother is registered at your address in their records. If he is write to them asking them to state in their records that he does not live there any longer. Good luck!