1 - See IRS publication 501, https://www.irs.gov/pub/irs-pdf/p501.pdf for requirements to claim a parent.
2 - The mother has a filing obligation in the U.S. I do not know the filing requirements for Canadian tax laws. Also, she is probably subject to the FBAR and FATCA requirements.
3 - Your situation is complex and will require a legal opinion. Since the mother failed to file taxes for 30 years and has never filed any FBAR or FATCA forms.
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