What a sad story. I'm sorry for your loss.
I'm sure we know so much more about diabetes now than we did in 1953, but diabetes is not usually a disease that requires an immediate interaction or insulin injection. The number of insulin units a person receives should start smaller, and then the doctor should increase them gradually, if needed.
Of course, the early insulins were derived from animal products, and who knows what else. It's possible your cousin had some kind of allergic reaction.
No, I think it was negligence, or incompetence. My cousin was sent home from school for vomiting on Wed. The doctor refused to see her. By phone he said she “probably had polio”. My uncle had her admitted to the hospital on Sat. The hospital made the diagnosis. The doctor (who was now there) ordered the insulin and then rushed off to a business appointment. My uncle begged him to stay because she didn’t look good. She died an hour later. The attending nurse is the one who said that the dose was too high.
My aunt and uncle did not sue because you didn’t sue your doctor in 1953. They had known the doctor all their lives. This doctor had a record of slipping out the back door when one of his patients was dying and refusing to face the family. But, my aunt refused to hear a word against him for the rest of her life. They did change doctors, however.
I was 2 years older and remember how hard it was for Karen to recover from that tonsillectomy. She was sick for weeks (end of the summer), and I never saw her again. She lost a lot of weight, but everyone thought it was just because she was a budding teen ager.