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The first thing you need to do is report the mother to the police for theft if she left the trust fund to your wife. The IRS will need proof of illegal action by the mom to get you out from liabilty for taxes. You will also have to sue the mother for breach of fiduciary duty, as it is likely she was the trustee which is how she emptied it and if she was not the trustee you would need to file suit against whoever the trustee was for violating the trust and allowing the mom to get the money out.
You will need to file both the criminal complaint against mom and civil suit and you will need to provide this evidence to the IRS to avoid being liable for the taxes due.
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