Who is The Armstrong Family from Murder on the Orient Express and what is their importance?

The Armstrong family has been victimized by the criminal Cassetti. Cassetti kidnaps little Daisy, and she is later murdered by Cassetti and his gang. After Daisy's body is found, her mother Sonia, who was pregnant at the time, went into premature labor. Both mother and infant died. As a result, Daisy's father, Colonel Armstrong, shot himself out of grief, and their maid, who was mistakenly implicated in the kidnapping, killed herself. Cassetti is thus, directly or indirectly, responsible for five deaths.