Whether your brother was acting as your mother's attorney as regards ***** ***** of the demand note is a question of fact for the trier of fact to determine in the event that there is a lawsuit over the issue. In that event each side will have the ability to present facts and argument. A one sided argument is useless in determining the outcome of a matter that could go either way, depending on the facts. Regardless of the facts and speculation that you offer, it simply is not any help in my coming to some sort of reliable answer that you clearly are seeking.
You wrote: at the time of the loan he was also acting as executor of my father's estate wherein my mother was the sole heir. This is entirely unrelated to whether your brother acted as your mother's attorney in regards ***** ***** loan document.
The note states it is to be secured by a deed of trust which would have had to be prepared by him although there was no deed of trust prepared. The fact that no deed of trust could suggest that your brother was not hired by your mother to handle the paperwork treated to the loan. Unless your brother was a seasoned real estate attorney, why might he not have hired independent counsel to prepare a complex document like a deed of trust? A demand note is a basic legal document unlike a deed of trust. Or on the other side of the coin, an argument could be made that if he was her attorney then he failed to provide the services he was retained to provide. It is impossible for me to determine which.
He also did my mother's taxes and has a power of attorney. That does not prove, or even suggest, that he was your mother's attorney for purposed of the loan.
He also gave legal advice to my mom regarding her real properties. All this proves is that he has acted as her attorney in past circumstances----which is to be suspected as between a lawyer son and a parent in need of legal assistance.
I'm sure if my brother wasn't an attorney my mom wouldn't have lend this much money without having an attorney look at it This is simply speculation on your part, unless you have some evidence to support such a statement beyond your gut feeling.
What I am telling you is that it is not possible for me to rule as a matter of law whether your brother was or was not acting as an attorney for your mother in the loan process.
If you want to pursue this matter then I suggest that you consult with a local legal malpractice attorney who has the ability to file suit, engage in discovery and perhaps gather enough facts to be able to make a reasoned judgment as to liability. From where I stand, it could go either way and nothing short of a confession by your brother will sway my thinking here.
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I wish you well,