Accommodation arrangement with brother.

Hi All,
I am going to apply visa for my wife. I live with my brother who own the house and I pay council tax, electricity bill, gas bill, phone bills etc. We don't have any official or paperwork for living together. Could you please advise me what I need to prove for my accommodation and arrangement with my brother?
I read somewhere that there is a letter required from my brother? could you please mention what I need to include in my letter or any sample letter would be really helpful. I am already going to submit house inspection report . But please let me know if any other document I need to submit with her application.
Many thanks