As far as getting good health is concerned ,it is not posdible till the blood sugar levels are controlled properly.These levels of blood sugar levels are very dangerous.The reason it is dangerous is that stresses in the body get multiplied at such a high range.
Infections, poor eating, stress from the environment, stress from activity, stress from psychological aspects, and many other situations may result in such high blood sugar levels and fluctuations.
These levels of above 300 involve the entire body and endanger all the organs and systems.
Ideally your mother in law should be admitted in the hospital and her sugar levels be controlled by getting proper doses of long acting and short acting insulin
She should also be investigated for the cause of fever
and treated accordingly.
You must make it a point that her blood sugar levels are controlled properly to avoid future micro as well as Macro vascular complications.
Along with insulin due attention should be paid for her diet and all dietary precautions and restrictions be observed.
She should also be encouraged to do regular aerobic exercises