Daughter of mom disagrees with MD's advice for mom to get 2nd biopsy opinion. Mom wants it. How to deal with such a loud and unfair daughter?

Second Opinion. Ignore her. As long as Mom is within her right mind, she has a right to make her own medical decisions. A second opinion on a biopsy is not something to trifle with.
Consider motives. If Mom is competent for medical decision making, the daughter has NO legal right to be involved. I would seriously consider whether or not this child should be given medical power of attorney in this case. Many children sadly want the inheritance money more than they care about the welfare of their older parent. This is sad but have a wealth of experience in this.What's your relation 2 patient?