There is no such thing as claiming a dependent for only six months --- you either claim them for the tax year or you do not claim them. Since your sister claimed the children on her return, their Social Security numbers were put into the system and now you cannot e-file your return or claim the kids yourself unless you print, sign and mail your return. Eventually the IRS will require more information from each of you to sort out who could claim them for 2020.
The IRS goes by where the children spent the most nights in the tax year. Who had them for the most nights in 2020?
Please explain "I didn't have them all year."
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