I had tended to side with the mother-in-law until I read this bit:
"It is tragic that you have diabetes. However, you aren't the only young person in the world who is a diabetic. I know quite a few young people who have this condition, one of whom is getting married in June. I have never heard her discuss her condition. She quietly gets on with it. She doesn't like being diabetic. Who would? You do not need to regale everyone with the details of your condition or use it as an excuse to draw attention to yourself. It is vulgar.
As a diabetic of long standing you must be acutely aware of the need to prepare yourself for extraordinary eventualities, the walk to Mothecombe beach being an example. You are experienced enough to have prepared yourself appropriately."
That does seem rather unsympathetic to put it mildly.
Geoff Endacott