I got two of those letters for people I knew and had them in my email address books. I also figured out that it had to be from facebook because when I joined facebook I saw that they had accounts. Actually I think Facebook needs to take some responsibility in at least involving the FBI in this.
What about your mother's bank didn't they question such a large check our money transfer? Hopefully something can be done about this but in any case it's time for you or your berother to get financial power of attorney.
So sorry this happened, I will say a prayer that this all works out. God bless.