Tax Reform Update: Personal exemptions are suspended for tax years 2018-2025.

Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 26:

Overview of the Rules for Claiming  a Dependent

• You can’t claim any dependents if you (or your spouse, if filing jointly) could be claimed as a dependent by another taxpayer.
• You can’t claim a married person who files a joint return as a dependent unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid.
• You can’t claim a person as a dependent unless that person is a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.
• You can’t claim a person as a dependent unless that person is your qualifying child or qualifying relative.

Tests To Be a Qualifying Child

1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year and younger than you (or your spouse, if filing jointly), (b) under age 24 at the end of the year, a student, and younger than you (or your spouse, if filing jointly), or (c) any age if permanently and totally disabled.
3. The child must have lived with you for more than half of the year.
4. The child must not have provided more than half of his or her own support for the year.
5. The child must not be filing a joint return for the year (unless that return is filed only to get a refund of income tax withheld or estimated tax paid).

If the child meets the rules to be a qualifying child of more than one person, only one person can actually treat the child as a qualifying child.

Tests to Be a Qualifying Relative

1. The person can’t be your qualifying child or the qualifying child of any other taxpayer.
2. The person either (a) must be related to you in one of the ways listed under Relatives who don’t have to live with you, or (b) must live with you all year as a member of your household (and your relationship must not violate local law).
3. The person's gross income for the year must be less than $4,200
4. You must provide more than half of the person's total support for the year.

Page 32: Relatives who don't have to live with you. A person related to you in any of the following ways doesn't have to live with you all year as a member of your household to meet this test.  

• Your child, stepchild, foster child, or a descendant of any of them (for example, your grandchild). (A legally adopted child is considered your child.)
• Your brother, sister, half brother, half sister, stepbrother, or stepsister. 
• Your father, mother, grandparent, or other direct ancestor, but not foster parent. 
• Your stepfather or stepmother. 
• A son or daughter of your brother or sister. 
• A son or daughter of your half brother or half sister.
• A brother or sister of your father or mother.
• Your son-in-law, daughter-in-law, father-in-law, mother-in-law, brother-in-law, or sister-in-law.

Any of these relationships that were established by marriage aren't ended by death or divorce.

Foster child. A foster child is an individual who is placed with you by an authorized placement agency or by judgment, decree, or other order of any court of competent jurisdiction.

You may also wish to utilize the IRS Interactive Tax Assistant (ITA) topic Who can I claim as a dependent?