Nobody - not even your mother - are getting fat out of thin air, so there must be some "hidden variables" into this...
When's the last time she had a physical? She is approaching or in menopause. A sift in hormones can lead to weight gain and/or a hard time losing weight. It can also negatively impact sleep. Also, as women age they have an increased chance of thyroid issues.
Other things to consider include:
1. Vitamin D levels
2. Iron levels including ferritin
3. Blood sugar