I just finished listening to this amazing episode of This American Life about two babies who were switched at birth and didn't find out FOR MORE THAN FORTY YEARS even though one of the mothers knew all along.
On a summer day in 1951, two baby girls were born in a hospital in small-town Wisconsin. The infants were accidentally switched, and went home with the wrong families. One of the mothers realized the mistake but chose to keep quiet. Until the day, more than 40 years later, when she decided to tell both daughters what happened. How the truth changed two families' lives -- and how it didn't.
The worst part about the whole thing is that the mother that knew, Mrs. Miller, always treated her non-biological daughter differently, like she wasn't really a full part of the family. The Millers sound like awful people.