I wouldn't waste my time asking on the expert forum where japdip told you to go. (sorry, japdip!) But you know they will not say anymore than you. The best explaining doctors come to this forum and they are not from the Cleveland clinic.
Your sister in law has now progressed to stage 4 cancer, and like japdip says, it is not curable. It is not good, that's for sure.
But, I also agree with japdip, that it is not necessarily the end at all. The bone cancer can be treated, the pain controlled, and many women live for many years with stage 4 bone cancer, and for some the quality of life is very good.
I also do agree with japdip that the information, and 'thinking', and prognosis, etc, should come from your sister in law's oncologist and not from you or anything you hear here. A good reason for that is already that nobody knows for sure how well your sister will do, and/or if she will develop other complications, and when, and how many.
What we are trying to tell you is that the situation is not hopeless.
The best place to get a true picture that would deal specifically with your sister-inlaw's case would be from her Oncologist. Bone cancer that has been determined to be mets. from breast cancer is one of the most treatable types although no metastatic cancer is cured. It would be my choice .... that no doubt sounds odd but I think all who have survived the initial breast cancer have thoughts of future mets. You might (if you haven't already) post your concers on the "Expert Forum" to get an opinion from the Cleveland Clinic Physicians. Regards .....