If the Power of Attorney is for all general purposes then the attorney in fact could indeed use that power to transfer real property of the principles.  But if that transfer would be considered an abuse of fiduciary power or a breach of fiduciary duty the brother/son may indeed have a claim here.  I think that you are intimating that the transfer was for personal gain or for the gain of her family member and not really sanctioned by the parents.  Then the brother/son needs to seek legal help to remove her as a fiduciary (POA) and to set aside the transfer of the property.  I is not going to be easy.  Good luck.