I am from a family of five. My sister and one brother are co-executors. My parents of 83 years old have finally completed their will and trust. My other two brothers have recently persuaded my parents to take a loan on one of their rental properties (but it will be in my parents name only). At the time of their departure how can we proof this was a lona to my brothers?
Thank you for any help

Have the brothers sign a document attesting that this loan was for their use and any balance at the time of your parents death would be deducted from the brother's inheritance.

No, of course they don't want to. But, my mother has a signed, notarized letter to the executor stating that the loan was for my brother's personal use- will that be good enough?

Who signed the notarized letter? Your mother? If so it's her word about what happened.

She needs a signed agreement with your brother, also notarized, to make this a debt payable to the Estate.