What do I need to do to become a health surrogate

I am the primary caregiver for my mother-in-law, but my brother-in-law holds the health surrogate. He is not in any way involved in any medical decisions, only me. I handle her doctor appt, meds, etc. I need to be able to talk with insurance companies, etc., but they say I need to be the surrogate. What should I do to protect her and me?