Every family has secrets. If you think yours is exempt, you're probably just not snooping hard enough (may I recommend Ancestry.com). But sometimes, if you want to find out your family's best-kept secrets, all you have to do is wait until you reach adulthood and your parents deem you mature enough to know the truth (even then, you might need to wait until your mom has had enough wine).
In a popular Reddit thread, someone asked: "Did your parents reveal a secret to you once you reached adulthood? If so, what was it?"
1.) From Dervrak:
That I had a much older half-sister. Apparently my father had got some girl knocked up in high school, her parents didn't like him and thought they were too young to raise a kid, so they just packed up and moved. He knew she existed, but never tried to locate her and just moved on with his life.