...
Son, daughter, or foster child
Brother or sister
A descendant of any of these
There are additional rules for foster children.
2. Age
Age To meet the age test, the child must be:
...
To meet the joint return test the child must not have filed a joint return for
the tax year they are being claimed.
The only exception to this is if the dependent and their spouse are not required to file a tax
return and file to claim a refund only.