Does a personal letter hold up in court?

About a year ago my father ped away. At the time, he left my brothers and I his life insurance and his 401k as beneficiary's. My stepmother his wife was the left the house. Somehow my stepmother was able to borrow against my Dads 401k and also cashed out about $12,000. Don't ask me how but this was done before the account was transferred into our names after his death. After the account was transferred to us, she wrote my brother who is executor of my Dads will in a personal letter about what she had done and that it was an accident, that she didn't realize our father had left that to us. In this letter she stated that she owed us approximately 30,000. She promised us in the letter that she would pay us back after she sold my Dads house. The house has since sold and she is literally trying to "run with the money". She won't speak to any of us. Legally does this letter have any chance of holding up along with the account statements that show what she cashed out?