
Can anyone spoil me if this is gonna be a sibling relationship or are they actually going to be in romantic relationship? Cuz I really hope they stay in a sibling relationship, I might drop this if they don't.(╥﹏╥)

They're going to be in romantic relationship
But in my opinion, they're not siblings because :
1. FL was kidnapped by ML's mother, FL has adult mind with full knowledge that ML is not her brother
2. ML most likely also knows that FL is not his sister... It's so weird that FL managed to survive (despite almost got killed in the flashback), it's also weird that people think she's the real daughter when the corpse of the dead mother is obviously still pregnant, i haven't read the novel but in my opinion ML might save FL from getting killed (since ML has strong power i'm sure he can defeat the killer), most likely ML is also the one who lied to everyone that FL is his sister so FL can stay alive in the family
3. There's no parent figure who raise them as siblings, they grow up on their own, and they might not see each other as siblings

yesyes same thought.. There are legitimate reasons as to how a sibling bond can be created, such as a set parental figure and the latter. I'm not defending, by all means, incestal relationships whether blood related or not, I'm just saying the author has reasons as to how their relationship will grow and I respect that and actually intrigued by it.. Anyways I hope this isn't seen as an attack to the original comment, I completely respect your opinion hehe (=・ω・=)

actually, this is a bit of a spoiler, so I'll warn you right now if you don't want to know :
There's actually a requirement between Dietrich(ML) and his shadow demon. He has to kill the one he's most attached to - right now, him tolerating and actually caring for the FL is because he needs to build that attachment and then kill her, so he can have full power and be able to become the head of the Lagrange. If I'm not wrong that's why the FL died before the MC reincarnated in her body. I didn't manage to find a good place to read the novel and didn't get to read much past where we are now in the manhwa, but I do remember seeing that detailed in spoilers when I looked for them.

No that's not what I meant. He HAS to kill her if he wants to be the head of the Lagrange because it's the only way he can fully gain the power of his shadow demon. I'm not entirely sure why she died in the book that the MC read but right now she's definitely going down the route of death without realizing it. Unless ML chooses not to kill her and therefore doesn't become the head of the Lagrange she won't live
Ummm can I just have Banam please? And rix honey please staph you have nuah already