[Legal Question]Changing Legal Guardianship of an Adult?

If any of the legal types here could help or at least point me in the right direction I would really apprecaite it.

Here's the situation: My half-sister, who is 20, is going to be living with me during the summers while she is off from school because she can't stand living with her mother or step-father. Her step-father's insurance/job situation is really flaky and currently she doesn't have insurance. However, I do have family insurance for myself, my wife and son. The benefits office here at work (they weren't very helpful) told me that if I were her legal guardian, I could bring in the paperwork and put her on my insurance. The best part of this is that I won't have to pay extra since I have the family plan and it covers all of my dependents.

So how do I go about this? Since she's an adult can she just give me power of attorney or guardianship herself?

Thanks in advance.

 

Thanks for the reply.

I don't think I want to go that far, unless I have to. I also want to minimize cost (obviously).