I would suggest looking at what is happening during the time of these blood sugar spikes, for instance, what was she eating, how much insulin was she taking, etc. My guess is that it would have to do with the amount of carbs and insulin, specifically, too many carbs and too little insulin.
She could have a condition called diabetic gastroparesis. This is where the nerves to the stomach are affected. Some medications may help along with liquid diet. A cgm (continuous glucose monitor) may be very useful if her insurance can cover it . Best wishes to your sister.
Oh no, so sorry to hear about your sister! Has she changed any habits or encountered any stress that might cause her sugar's to be so off? How about her medication, have they tried to adjust that? How's she doing with monitoring? If they are not giving you guys any ideas on what's happening, are you able to go to another doctor for another opinion?