My step mother is my fathers power of attorney, I want custodial guardianship, she is denying him care.

How do you know that your father wants medical care? How do you know he did not say that he is ready to die?

You may be perfectly right. Or you may be mistaken. But you will need to hire an attorney to attempt to get guardianship, and it will be good for you to have very convincing evidence.

Is there a written advanced health care directive?

If you cannot afford an attorney consult, call you county Bar association for a referral to a lawyer who specializes in this area. Frequently lawyers will give a free consult to those who come thru the Bar referral line. If your Dad's attorney was able to assess Dad was competent to make up POA and living will, you may have to accept the choices he made. You might find a counselor who could see
Dad and assess his competency at home. This might be beneficial in event Dad might want to make a change in these documents now. There are lawyers who took special training in medicine to be able to assess competency from med as well as legal perspectives-these "specialists" would be helpful to you.

Have you and your father discussed his wishes in the past? Or are YOU wanting certain health care for him? These are very emotional and difficult issues and discussions to have, but especially in this case,very needed!!! Even those of us who thought things like this could never happen in our family, have found out otherwise, after a certain family member is no longer with us. I am not saying that what you said is not true. But your Dad may also have changed his mind on certain medical treatment. If you are able and he is able to mentally have this conversation, perhaps with the help of a counselor or mediator, you could have a conversation with him about this. You could just make a "dinner" appointment with him. Tell him you would just like to have some "daughter" time with him. Hopefully, your step mom will not deny you this.