I think you have done enough for this unappreciative woman, but, on the other hand, if you withdrew your help and something happened to her like a fall, you would feel really bad! It's a tricky one! I think you should gradually cut down on what you do for her and explain that you will have to be spending more time looking after your grandchildren, but that if she has a real emergency, she can always contact you. Having said that, I used to help an elderly lady who lived a couple of doors down, take her to doctor's appointments etc. as she was completely neglected by her two sons, who weren't very far away. She was very difficult and wouldn't listen to any helpful suggestions to make her life easier and was only interested in herself and never asked how our family were. The last straw was when she rang quite early in the morning and asked me to go round and remove a spider from the wall!
I was in my nightie at the time, so I told her to deal with it herself! On the other hand, it was a good thing we had a key, as when one of her friends came round to take her out for a meal, ands she didn't answer the door, my husband went round with her and they went in together and found the old lady on the bed, having had a stroke, and were able to call an ambulance. If your lady has sons, they should definitely be helping out more.