how do i gain medical power of attorney for my 22 yr old sister suffering from eating disorder?

If your sister will not voluntarily sign it over to you, you will have to apply to the court for an emergent guardianship.  You will need a doctor's certification that your sister is not competent to make her own health care decisions.  You will have to make the guardianship limited, for example you will not take away her right to vote, or her right to drive, but only to make her own medical decisions.  Depending on the county in which your sister lives, the Surrogate can be very helpful in guiding you as to what needs to be done, or you can contact an Elder Law attorney.  Even though your sister is not elderly, this is the specialty that does the guardianships, and knows how to get it moved through the system.  Good Luck !