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