Fortunately, the banks are on Grandma's side on this one. Unlike Bitcoin, banks will not let someone who obtains a few credentials simply transfer an arbitrary amount of money to anywhere in the world in seconds. Banks seems to have many layers of checking for potentially suspicious activity, and ways to reverse transactions that are later proven to be fraudulent.

Bitcoin has some interesting properties, but holding bitcoins directly is definitely not right for anyone who can't be trusted to keep their critical credentials secure no matter what.