The IRS recognizes that living situations can be complicated and that people often come to the aid of those who aren't directly related to them. For tax purposes, you may claim any legal relative under the age of 18 as a dependent child. Nearly anyone else may be a dependent relative -- regardless of any legal relationship -- if they meet all residency and support requirements and the relationship or living situation is not illegal under local statues.
Dependent Taxpayer Test
You cannot claim another person as a dependent if you yourself qualify as someone else's dependent for tax purposes. This is true whether or not that person actually claims you on a tax return. If you are married and file a joint return, neither you nor your spouse can claim a dependent if either of you qualifies as a dependent. Note that a spouse cannot be a dependent, even if one partner provides most or all of the family income.
Joint Return Test
In general, you can't claim a dependent who is married and files a joint return with someone else. If you are supporting a married person, such as a daughter, and she files a joint return only to receive a tax refund, you may be able to claim her as a dependent if she meets all other qualifications. In this case, neither your daughter or her partner could owe taxes for the year, even if they filed separately. As a couple, they must have no tax obligation for the dependent period.
Citizen or Resident Test
All dependents claimed on federal taxes must be U.S. citizens, U.S. residents, U.S. nationals, or residents of Canada or Mexico. If you legally adopt a child and that child lives with you for most of the year, you can claim him as a dependent even if he is not an official resident or citizen yet. This rule means that foreign visitors, such as exchange students, do not generally meet the requirement for dependency.
Children must live with you for at least half the year to qualify as dependent children. Other dependents must live with you for the entire year, unless they are a specific close relative, such as a parent, sibling, adult child, niece, nephew or grandchild. See IRS Publication 501 for a current and complete list of dependents who don't have to live with you. The IRS does not disqualify someone from residency for a temporary absence, such as a vacation, business trip or hospitalization, if the person otherwise calls your home her home.
You must supply at least half of the dependent person's living expenses and other financial support for the year. In addition, dependent relatives cannot earn more than the exemption value for the year -- $3,700 for 2011 -- unless they're disabled and receive income from a sheltered workshop.