The woman you are searching for is my great aunt. It is true that she has a brother named Oscar and that her fiance was killed in a tragic accident while harnessing horses. Gladys lived her entire life in Klemme, Iowa area (Oscar was the only sibling to move to Wisconsin).
Gladys died within the last 10 years. In later life she married, but off hand I don't know to whom. Her brother Harry died just last month at the age of 101.
If you are still in need of info, please reply and I'll contact relatives back in Iowa to get you the specific details.