It took years, and a lot of frustration, but in the end I learned that she will never change, and I don't deserve the aggravation.
What ended the whole thing was a planned "girls weekend" at our place up north. She and I, and our 2 cousins that we don't see often were going to spend the weekend together just hanging out. My cousins drove from their homes, a few hours away, to my home. She was supposed to come to my home as well, and we were all going to go in my van to the place. (No one else knew how to get there.) We were going to stop first at a local restaurant for supper and then hit the road. Cousins and I waited for over an hour at my house, then finally went to the restaurant, telling my hubby to send her there to meet us. Got a ph call from him saying that she was "running late" (by this time it was 2 hours late) and would we order some take-out for her. When we left the restaurant we had to go back to my house. She finally showed up almost 2 hours after that, and laughed about "Oh, you know me, I always run slower than you do, haha". Since at this point she was 4 hours late, we did NOT laugh along with her. No sense of humor, right?
We still had a 2 hour drive ahead of us, and she wasn't happy when I told her she had to eat her take out in the car on the way, that she had wasted too much of our time already. By the time we reached the place, it was late night and we had to unload groceries, bedding, make beds, etc. It would have been fun doing all that together if we hadn't already been pretty irritated and not tired.
That was my wake-up call. I no longer invite her to do things. If I do invite her, I tell her what time we are leaving and if she can't make it at that time she will have to drive herself. Or not go. It's a shame, but it's the only way to not let her ruin the things we do.
As I said earlier, she does make it to work on time, because they can fire her. And she makes it to church functions as well, but that's something that is close to her heart and is also an escape for her from her weekday routine. The rest of us are just chopped liver.