You can (and should) fill out forms for Power of Attorney. There are two .. one is for medical care and the other is for financial affairs. You can appoint anyone for either of these tasks so that in the event of your incapacitation they can make pertinent decisions on your behalf.
It would also be a very wise idea to make out a will, naming her as your beneficiary. You should both have wills .. ESPECIALLY if you have children so that care and control of your children can go to whom YOU choose .. not who the state chooses.
This is the person who asked! We are getting married next year, we just had our second child. During child birth, she gave us a scare. Her mom pretty much talk for me, and we honored all her wishes. Lets just say I dont trust my mom to let her decide. 15 year 2 kids I think my girl friend should be making the decisions. We have been engaged for 10. Reason is for Medical care. the state I live in New Hampshire: Common law marriages effective only at death. (N.H. Rev. Stat. Ann § 457:39).