Metastatic Lung Cancer

My mother-in-law was just diagnosed with Metastatic Lung Cancer. I don't know a whole lot about all of this. They say that it's in here lungs, bladder & overies. They are starting her on Chemo today. I need any info that anyone can give. We feel like the doctors aren't telling us everything that needs to be told. They said that she has arthritis in the back but they are going to treat it as if it were cancer. Why would they do that if it's just arthritis??? Any answers would be great!