Can a bone fracture ever heal in a bone cancer patient?

Means, my sister had breast cancer that metastasized to her bones, her bones would spontaneously break. They never healed.

What does the bone scan look like? My sister's bones were very deteriorated and there was nothing in them to heal them with.

I am sorry you are going through this. It is a difficult disease.

This should be discussed with the doctor who knows the patient. This is from one study:
" In 123 cancer patients with metastatic disease, 129 pathologic fractures of long bones were assessed to determine the rate of osseous union. Bony healing was observed in 67% of malignant fractures from multiple myeloma, in 44% of fractures secondary to metastatic hypernephroma, and in 37% of neoplastic fractures from breast carcinoma."
So, in general the answer is "yes, it can heal" but in reality for your loved one the answer depends on the patient him or herself; we are as individual medically as our own fingerprints.

What does the patients cancer doctor say about it? They will no better than any of us will.
Hoping for healing in whatever way it comes.