Yeah, same as if she wasn't yours. Until you put a ring on her finger and go to the JP and make it legal, won't work.
Loophole, you meet the State law as being married by common law 95 to 7 years and/or with your kids) . Then use joint return and suffer the marriage penalty tax.
I know that when you file for fininacial aid for college, the form asks about dependants and defines a dependant as a person who received over half their support from you. This could be a child, a spouse, or and elferly person that you are caring for, for example. I don't know how this works for filing takes but, I would think that the definition of a dependant would be the same.