Parker, you're doing all you can. I know it's SO hard, but your positive influence and your love will make a difference in your granddaughter's life. My family and I are in a similar situation with my niece. Her mother isn't the mother we would want for her and we worry a lot, but it's beyond our control. She has weekdday custody and my brother has his daughter only on weekends and some holidays.
So, my parents have my niece at their house as often as possible and I do the same and so does my brother... and we talk to her on the phone and send her letters when we can't be with her. We just want to make sure she has as much love and support and positive reinforcement in her life as possible. That's all we can do.