I had A SIMILAR SITUATION in the early 1960's with two related taxpayers.The broker stole their shares.
Both taxpayers were audited (by different IRS agents)one was allowed the casualty loss (theft) one was denied. No penalties were charged on the taxpayer who was denied the deduction, interest was charged.
You also have to consider the chance of being challenged (audited).
Again, this is not tax advice discuss with your tax advisor.