Does a new spouse have full power over the children (adult)?

My father who is 85 year old married a woman soon after his first wife passed. The new wife has medical POA. My father's health began to fail and now he is on life support in the hospital. The Dr. have said in a round about way that he will not recover. The new wife refuses to take him off support. My father is in agony and has been for almost a week. My sister and I both agree on letting him go. Is there any way to override his wifes authority?