Flowers in the Attic
My grandmother was born to a brother and sister and no one ever talks about it. She was adopted out and found out the truth later when her birth mother would come visit a lot. Dad found out later in life.
It was the 1920s, they were 12 & 14 on a remote farm—genuine Flowers in the Attic-type stuff. This was typed with my two fingers and one toe. Story credit: Reddit / schnesnchowIgo