I am very sorry you have to go through this again and that your mom has lung cancer now.
I beleive that we each have the right to choose rather to treat or not. You may just have to accept your mom's decision and support her any way you can.
However, it sounds like your mom's disease is not as far advanced as your dad's. If the cancer is just a few small tumors, they can treat it with targeted radiation such as Image Guided Radiation Therapy (IGRT) or cyberknife and then do chemo. Targeted radiation avoids collateral damage to healthy lung tissue.
Some people have conventional radiation and chemo in a situation such as your mom's and have a very difficult time with radiation damage to healthy lung tissue and pneumonitis, while others sail right through it.
If your mom had targeted treatment, the only difficulty she might have after that is dealing with the chemo. Most people get through chemo ok these days.
I wish your mom well and you too in the days to come.
Don M