I know firsthand what your girls are going through. I lost one grandma when I was seven, the other when I was ten right before Christmas, Aunt Barbara a year later right after Christmas, and Grandpa (the husband and father of the latter pair) a month after Barbara. I wasn't very close to the first since she was in the nursing home and basically unable to speak for as long as I can remember. I was very close to my other grandma and Grandpa and fairly close to Aunt Barbara. I didn't actually see Grandpa right after he died, but I had a dream where I watched him die and the details (time included) turned out to be on par with his death.
I'd tell it like it is without too much sugarcoating and not too much detail. Tell them that she's got the same problem as your dad and they'll probably understand. There's probably no need to tell them to make the most of the time they have left with her since they'll probably do that without being told. I made the most of my last vacation with Grandpa and even paid for our final dinner on the trip.
They'll never fully get over the pain of losing those they love, but it's something we all carry with us. They'll carry their good memories of the people they lost forever. The tears help to heal our hearts. Your daughters might be rather depressed for a time, but they will most likely get over it fairly soon if there isn't too much additional trauma (it took a while for me since there were the additions of Dad losing his job, Dad being diagnosed with prostate cancer and diabetes, puberty, my sister moving away, kids being cruel, my pets dying, and moving to another part of the country away from a lot of family). At least they have the church to help them through this; my family had recently left the church and I was unsure of my own faith (still don't attend church, but I prefer to worship on my own time and in my own way).
Please note: in my case, I knew most of what was going on with my dying grandparents since I grew up surrounded by the medical field and my mom is a nurse.
My condolences to you and yours. Best of luck.