Scared and worried

So,a few weeks ago,my best friend came to me almost crying and said that her mother had been diagnosed with Stage IV Advanced breast cancer with cancer cells in her liver and around her heart. Now,my best friend has a little sister about ten and a brother who is fifteen. We are both twelve,and I'm scared to death. I refer to my best friend's mom as my 'second mother'.We aren't idiots,we both know it's terminal. But I'm really close to my best friend,and this is like losing an aunt.I just don't know what to say.I'm not sure what's worse: having your mother die suddenly or knowing she will die soon. My friend often needs to go out of clas to comfort her little sister,wh just bursts int tears. Just knowing that soon she will be gone scares me.But I'm worried.What can I comfort her with?