It is likely you can claim her as a dependent, but not as your child.
You can claim your girlfriend's little sister as a dependent if you meet all of the following conditions:
·
You
provided more than half of their support.
·
They made less than $4,150
in gross taxable income.
·
They
lived with you for the entire year if non relative.
·
They are not a dependent on
someone else’s taxes.
·
They are not filing a joint
return with someone else.
They are a U.S. citizen, resident alien, national, or a Canadian or
Mexican resident.
For more information about claiming dependents, follow this link: Turbo
Tax on Dependents