In her first interview for international media, Anna told CNN of her family's shock and the guilt that she says "stings us on the inside."
"We ask ourselves the question every day: What happened, why? He was in the same womb as me. We grew up together, so how did our paths end up so far apart?"
Anna still can't believe the little brother she remembers could be capable of such horror.
"He was a nice person, a sensitive person, a bit shy but somebody you can rely on. He was generous, someone who loved to laugh and joke, a cheeky little boy," she said.
That cheeky little boy turned into a mass murderer after traveling to Syria to join ISIS, where he's thought to have spent the past several years.
When he left, he told his family he was going to the South of France with friends. Little did they know he would never be coming back.
Sitting in the small apartment where she grew up with Samy, their parents and a younger sister, Anna says her brother's radicalization began on the Internet.
The family had never been strict in their religious practice, but they noticed Samy's behavior change about four years ago.
"The changes from the way he used to dress, the way he used to listen to music, the way he used to go out, the way he used to express himself -- to laugh and to think as well -- that can't happen overnight, not by himself," his sister said.
