Your grandmother most likely does NOT have bone cancer, but her breast cancer has returned in her bones.
This is known as a metastasis.
Her treatments will be the same drugs used to treat breast cancer and will be very different from the drugs used to treat primary bone cancer.
She will now be classified as a Stage IV breast cancer patient with bone metastases.
Her cancer is not considered to be curable, but it is considered to be treatable.
Bone mets are among the "easiest" to treat.
Unfortunately, they can also be among the most painful.
Your chances of developing breast cancer are still the same as the general population.
If your mother should develop breast cancer, then she would be a "first degree" relative and your risk may be considered "slightly" increased.
If your mother developed the disease under age 50, then a test would be done to determine if there is a gene mutation.
Only 5% - 10% of breast cancers are considered to be genetically linked.
@Mildred f ... what an ill-informed answer! If breast cancer has spread, it is always Stage IV.
Bone biopsies are seldom done on women who have had breast cancer as metastatic lesions look VERY different than primary tumors, especially on MRIs.
A metastatic lesion will have a "splat" appearance on the bone whereas primary tumors do not.
Additionally, you are writing a death sentence for someone you know nothing about.
I have Stage IV breast cancer in many, many bones and I'm in "excellent" health.
ALL the women in the metastatic breast cancer support group I go to have been going for more than three years.
I'm the "newbie." Breast cancer treatments are so much better than they used to be.
Please ... don't answer questions for which you are not prepared to back up with facts.