The We Need to Talk About Kevin star revealed that she felt so resentful of her newborn baby brother that she tried to kill him.
"He was newly born and I was disappointed, because he was the third boy. That was enough as far as I was concerned," Swinton told The Telegraph.
The actress went on to reveal that she actually ended up saving her brother's life.
"I went into his room to kill him, saw some ribbons from a bonnet going into his mouth, and began to pull them out," Swinton said. "And I was discovered saving his life. So I had this strange reputation - my brother's saviour - and no-one knew I wanted to kill him."
Swinton recently suggested that she may retire from making movies, saying that she considers her acting career to be a major mistake.
The actress previously described her Academy Award for her role in Michael Clayton as "meaningless", claiming that the accolade meant nothing to her family.
Watch the trailer for We Need to Talk About Kevin below: