Large deposit into bank account

[V Krishna]

Hello,

I am in contract with an investment property and the under writer is asking for source of deposit on large deposits into my account 3 months ago. The deposit was from loan returned by my own brother that i lent him over the period of 6 years for his education. I have provided the statement from my brother with deposit amounts, however the under writer now wants the proof of statements when i lent him over the period of 6 years. The money was lent in small chunks several times and i dont have paper trail of every transfer. Would a letter from my brother explaining all this work ? Any other way to deal with a situation like this to prevent from loan getting rejected ? Any advice is greatly appreciated.