jam25,
I'm sure you are afraid of the reality, but you probably know in your heart how long your sister-in-law has. Since her cancer has progressed to bone metastases within only 12 months, and after all of the treatments, this is an aggressive cancer. But, even with aggressive cancers, it is difficult to put an exact time frame on it. Some patients may live for a few years, while others may continue the downhill slide.
Make the most of every day. Provide her and her family with as much support and love as you can. Do not delay any activities that you wish to do with her.