Yes, you can claim her (most likely, see full rules below) but then her husband would have to file Married Filing separately (MFS)**, which has higher tax rates. He cannot file single. If she had enough income to file, she would have to file MFS too. If you meet the dependent rules below, the best thing to do is for both couples to prepare their taxes both ways and see how the family comes out best overall, before filing,
A child of a taxpayer can still be a “Qualifying Child (QC)” dependent, regardless of his/her income, if:
1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled
2. He did not provide more than 1/2 his own support
3. He lived with the parent (or was away at school) for more than half the year
A person can still be an other dependent (Qualifying relative in IRS parlance, even though they do not have to be actually related), if not a Qualifying Child (QC), if he meets the 6 tests for claiming a dependent:
1. Closely Related OR live with you ALL year
2. His/her gross taxable income for the year must be less than $3,900 (2013) ($4300 for 2021)
3. You must have provided more than 1/2 his support
In either case:
4. He must be a US citizen or resident of the US, Canada or Mexico
5. He must not file a joint return with his spouse** or file a tax return claiming someone else as a dependent
6. He must not be the qualifying child of another taxpayer
**There is an exception to the requirement to file MFS. You can claim an exemption for a person who files a joint return if that person and his or her spouse file the joint return only to claim a refund of income tax withheld or estimated tax paid.
