Can I claim my niece on my taxes. I been supporting her for years

It depends. You may be eligible to claim your niece as a dependent if she lives with you and if no one else can claim her. She'd be considered your qualifying child.

If your niece's parent(s) live with you, she'd be considered their qualified child as well. If that’s the case, you will only be able to claim your niece as long as the parent(s) Adjusted Gross Income  (AGI) is lower than yours. See the IRS rule.

To determine if your niece is your qualifying child, the following questions must be true.

For a Qualifying child 

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

You may be eligible for the following tax benefits if you claim your niece as your dependent.

• Head of Household filing status
• Earned Income Credit (EIC)
• Child and Dependent Care Credit
• Child Tax Credit
• Medical expense deduction
• Education Credits

For additional information, review the TurboTax article Rules for Claiming a Dependent on Your Tax Return.