It is not normal for her to sleep for such long hours. Sleeping for such long hours would mean a hidden condition that she suffers. Check out whether she displays other symptoms like eating less or some signs of depression. You should also go to a doctor and ensure that he gets the extent of your mom's sleep issues.
being 90 yrs old is not normally healthy. she's pushing the envelope. God bless her and enjoy the moments you have. My mother in-law is 83 good heart lungs. brain is shutting down (mini strokes Alzheimer's )