There was a case here in Australia which made our news yesterday. Similar kind of thing, except the guy told the elderly lady he was from the Tax Office and needed her to send money, then her refund would be released. He also told her not to tell her family, she actually sent several "lots" of money via bank transfer. I think the total sum was around $50,000 (she had even taken out a $15,000.00 bank loan).
Seems these ratbags are the world over, best change your Mom's number to a silent/unlisted service and have your brother or yourself co-sign her banking/financial transactions. This may be more comfortable for her than giving over total control or power of attorney.
Sorry this happened to you and yours and I hope these disgusting thieves get their punishment in the foulest way possible.