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Each member of the Guaranteed Investment Certificate (GIC) is taxed on his or her share of the income. I am not sure why the mother of the nonresident member is paying the tax on her tax return.
The rules for nonregistered CIGs state that as of January 1, 2008, non-resident tax is not deducted from the interest on non-registered GICs. The investment company IS REQUIRED to issue a NR4 tax slip to report the interest to each nonresident owner. That nonresident owner should file a Canadian nonresident tax return.
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