Seeing loved one's sick/ill can be depressing, and seeing one's parents in a very unhealthy state is much more unbearable. It is a dreadful sight to see them so helpless.
This is a prayer you can make for the health of your parents:
Dear God, I commit my parents (father/mother, or both, mention their names) into your hands. They are my earthly parents but you are the parent to every one of us, including them. Please take good care of them as you do for your children. May they prosper in good health of mind and body. May your grace abound with them to stay hale and hearty and may the joy of the Lord be their strength and keep them strong, Amen.