Oh man.. This legitimately scared me when I read it. Like the MC being so jealous of her twin brother to the strange air around them when they made eye contact. The twin brother seemed like the golden child to me so I just took it as jealousy from the MC over the partial treatment from their dad. Then when she made some kind of deal with the devil, the twin brother turned into some creature so I assumed that it was her fault but apparently not. We see the flashback of the nanny who looked after them as babies, it was revealed to us that there was something wrong about the twin brother. Like he would just stare at the MC without making a fuss (like babies tend to do?). One incident that scared the maid was when she was consoling the crying baby MC then her sucking her thumb, then the twin brother following suit sucking his thumb right after. Man that scene creeped me out. Also it seems to be a hereditary thing amongst the MC and her family since her mom was the first one to make a contract with the devil. Like she brought these weird statues of demons or coffins when she got married with their father (when she left her castle to move to her husband's castle). The twin brother is a demon from birth it seems.
The Devil