Can I claim my 6 month (as of february 2017) niece as a dependent?? She does not reside with me but I do support her more than 50%..

Why aren't the parents claiming her?

Do the parents have to file a tax return?

Yes, you can claim her.

A qualifying relative must meet all five of the following conditions:

 - Relationship: The person must live in your home for the entire year (be a "member of your household") or be related to you.

 - Gross Income: Generally, it must be less than $4,050 (not including Social Security or welfare.)

 - Support: Generally, you must provide more than half the person's support. Special rules apply for children of divorced or separated parents or children receiving support from two or more people.

- Marital status: Generally, a dependent cannot file a joint tax return with a spouse. The only exception is when the dependent files a joint return only to get a return of taxes paid and, if separate returns filed, neither dependent or spouse would have a tax liability.

 - Nationality: The person must be a United States citizen; or a resident or national of the U.S., Canada or Mexico. An adopted child that is not a US citizen or resident of the US, Canada, or Mexico must live with you for the entire year.

Will I need any documents or form to send to irs stating she's my niece or/and that I support her...

@TurboTaxPeterM  I am sorry, but I must disagree with your unconditioned "yes" answer.   (It might be correct, but there is not enough information given to make that determination.)

Per the IRS there are 4 tests for a Qualifying Relative:
<a rel="nofollow" target="_blank" href="https://www.irs.gov/publications/p17/ch03.html#en_US_2016_publink1000170933">https://www.irs.gov/publications/p17/ch03.html#en_US_2016_publink1000170933</a>
Qualifying Relative
Four tests must be met for a person to be your qualifying relative. The four tests are:
   1) Not a qualifying child test,
    2)Member of household or relationship test,
    3)Gross income test, and
    4)Support test.

You left out the first test (Not a qualifying child test).
If the child *can* be the Qualifying Child of the parents, the child cannot be a Qualifying Relative of anyone else.

If the parents *can* claim the child, even if they choose not to, then nobody else can claim as a Qualifying Relative.  The parents "cannot allow" someone else to claim, the tax law does not permit that.  Only if the parent is not required to file a tax return is it permitted.

Okay so in my situation the parent doesn't have to file a tax return base on gross.. She already checked

Then your fine, as long as the following is true (but be careful- if the parent isn't required to file but does file to get a refund of tax paid *and* claims the EIC or other credit, then the *only* reason for filing was not just to get a refund of tax paid, it was also to get the EIC or other credit, so your claim could be denied):

Per the IRS
"Child of person not required to file a return.   A child isn't the qualifying child of any other taxpayer and so may qualify as your qualifying relative if the child's parent (or other person for whom the child is defined as a qualifying child) isn't required to file an income tax return and either:
   1) Doesn't file an income tax return, or
   2) Files a return only to get a refund of income tax withheld or estimated tax paid.