Stage 4 breast cancer

Hi everyone thank you for taking the time to read this. My partners sister found out two weeks ago she has breast cancer. It was a shock to us all but we was being positive until today when her scans showed it's already spread to her sternum and lungs.

My partner is devastated and I'm not sure what to say or do! This feels like a death sentence and she's only 35!

I know it can't be cured but what are her chances of living a long time with treatment?