why is it so hard to make a durable attorney pay back money?

I am the executor of my moms will and I have the last three years of cancelled checks, bank statements, payments to nursing home, withdrawals from financial firms of my brothers actions while our mother was in a nursing home. I found that he wrote $ 7,838.79 in checks to himself and his ex wife and other brother. And i see missing money from a financial firm. Why is it so hard to have him show receipts for those checks ? I retained m mothers lawyer and he said I will have to live with it. He said It will cost too much $ to fight him out of what is left in the estate. The form he sign was terminated in '09 due to this kind of abuse.