The mother asked the demon that grants wishes for a child. She gave birth to twins, originally a female human baby which probably she had already (maybe she stillwished for a child without knowing) and a demon looking baby. She was scared of duke (?) *from what I understood* because she had to birth a son for heir. Now desperate right after giving birth she looked for the demon again and opened a casket. She wished for a "son" not just a child but a son who will become an heir and the demon granted it. The demon looking baby is now a complete replica of the baby girl. The male baby was a peculiar one and doesn't act like a real baby but mirrors the female baby's actions. They are not siblings. Johann was made from some form of demonic magic and she was a child of the duke and her mother.
Putting aside the whole incest thing people are talking about, my problem is mostly about what would happen if they got caught, i haven't really read the recent chapters but from my understanding if they get caught only the girl would bear the consequences. Idk i just don't like the brother since from my understanding he is leaving most of the weight of their relationship on the girl. Like this whole chapter was her being afraid and worrying, but she cant lean on her brother at all. anyway i may be wrong since i haven't been paying much attention.