The CRA requires ex-spouses
information for a number of reasons, like eligibility for certain credits or
deductions, handling property you still own together, etc.
You may be able to find your
ex-spouse's SIN number on previous tax returns, or on other documents such as
court filings. You can also contact the CRA and try to get the info from them:
https://turbotax.community.intuit.ca/replies/2647704
Some people put in their
ex-spouse's SIN as 000 000 000 so that they can complete their returns, but
that could cause problems when filing, or when the CRA looks at it.
I’m separated or divorced. Why am
I being asked for info about my former spouse/partner?:
https://turbotax.community.intuit.ca/replies/3425400
What do I do if I don’t know my
spouse’s income?: https://turbotax.community.intuit.ca/replies/2647950
