Can I claim my nephews if the moved out of state in the middle of the year? And can someone else claim them for the second half of the year?

[Critter]

A dependent can ONLY be claimed on ONE tax return per tax year ... so who did the child live with for MORE than 1/2 the year ( + 183 days ) ??

Who can I claim as my dependent?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2018 taxes as long as they meet the following requirements:

Qualifying child

• They are related to you.
• They aren't claimed as a dependent by someone else.
• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They are under the age of 19 (or 24 for full-time students).
  • No age limit for permanently and totally disabled children.
• They live with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year.
• They made less than $4,150 in 2018 ($4,250 in 2019).
• You provided more than half of their financial support.

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them.

Related Information:

• Does a dependent have to live with me?
• What does "financially support another person" mean?
• Can I claim myself or my spouse as a dependent?
• Can I claim my parent as a dependent?

[DoninGA]

There is no claiming a dependent for a partial year.  The dependent is claimed for the entire year by whomever is eligible to claim them as a dependent.

There are two rules to claim someone as a dependent on a tax return, either the Qualifying Child rules or the Qualifying Relative rules.  If no one is eligible to claim them under either of the rules then they cannot be claimed as a dependent.

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, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.

To be a Qualifying Relative -

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,200 (social security does not count) in 2019
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.