I'm a criminal defense attorney, and former prosecutor, in Florida so I think I can help out.
As you may know, the progress of computers and the internet has caught law enforcement a bit flat footed. Slowly but surely, law enforcement is catching up with technology. At this time, I am aware of no Florida law that makes it a crime to hack into someone's email account, rather, the crime occurs with what happens next. For example, if you take passwords and utilize that to empty someone's bank account, a theft of those funds has occurred.
It sounds like your situation is similar. I doubt the authorities would prosecute your niece for having entered the AOL account. I say this for the reasons stated above and the fact that it is very difficult to prove, beyond a reasonable doubt, how committed the hacking. For instance, even though you may be able to trace it to a particular computer, you still can't prove WHO was sitting at that computer.
As to AOL, contact them and change whatever is necessary in your account to prevent any further access.
As to any monies, if I'm understanding you correctly, no funds have been removed as of yet. If I'm wrong and funds HAVE been removed, contact the local law enforcement agency in the jurisdiction in which the funds were removed. As I stated earlier, this is theft and SHOULD be prosecuted.
As to your niece's conduct towards your father, it may be considered criminal as Florida has laws on the books making it criminal to take advantage of the elderly. Again, if this has happened, contact local law enforcement.
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