In your brothers case from the short information supplied by you following steps can be taken in order to set up defence-
1. If your brother and the money lender is resident of Maharashtra, lodge FIR and a complaint under the provisions of the Bombay Money Lenders Act,
2. Lodge FIR with the local Police station setting forth the necessary details after consulting your Advocate.
3. Inform the Bank to stop the payment of the chequues which your brother claims to have given in Blank Signed Condition. Of course the bank would issue such return memo only if you have sufficeint funds on such account.
4. Issue a notice informing the correct state of affairs to the money lender and demand the cheques back from him.
However, for more specific advice your documents and more information would be required.
Adv. Amit A. Mahadeshwar