@bunny-k1m Hope this helps!
Dietrich and Anisa aren't related by a single drop of blood.
Anisa was born into the Euclids, who reside in the South. Her biological brother is Herman, who's the original male lead of the book she got isekai'd into.
The Euclids are the sworn enemies of the Langranges, who reside in the North. Dietrich was born between the union of a purchased slave named Camille and the Duke of the North, Derek, who has many wives. Dietrich is the villain Herman seals away in the original novel.
Camille kidnapped Anisa when she was a baby, who was hated by her birth family anyway for having red eyes (a trait common in the North, not so in the South, thus considered cursed by the South). Instead of rescuing her, the Euclids sent assassins to kill Camille and Anisa. Anisa somehow survived, Camille did not. But because Camille was pregnant, people assumed Anisa was her daughter, hence why Anisa is with the Langranges now.
So Dietrich and Anisa are essentially step-siblings.