I’m not sure if this article says more about banks’ reluctance to honor legal documents, the need for siblings to maintain their relationship, or the need for more careful estate planning in the form of a trust rather than a simple will and power of attorney. In this case, if the brother had a trust set up his sister may still have faced hurdles because of identity issues, but in general banks give much greater deference to trustees than agents holding powers of attorney.

Banks tend to take a defensive view of people with power of attorney for fear of litigation. Any discrepancy will gum up the works.
NYTIMES.COM|BY PAUL SULLIVAN