Bish what.... I'm on chap14

I can give you a spoiler if you'd like?

Sure gimme gimme

So, it's kinda complicated
When their mother was pregnant with Annette, she prayed to the devil for a son, so sometime during her pregnancy with Annette she conceived Johan, but when Johan was born he came out as an actual demon, like with a tail and horns and everything, so she made another wish to the demon for him to be human. So they are from the same womb, but they do not have the same father and they're technically not even the same species, hence it's up to you to decide whether you consider them siblings or not.
P.S: there was also a line that implied that the demon actually impregnated their mother; "Nine months after the demon arrived, the lady gave birth" or something like that.

So, on one hand; they were raised as siblings, they came from the same mother, they were born on the same day, and they acknowledge each other as siblings, so that's enough to qualify them as siblings in my opinion, but for some, due to the fact that they aren't the same species, and didn't come from the same sperm(father), that's enough to regard them as not siblings. So you should just take your pick of which ever theory you agree with.

I see thank you for the spoiler

You're welcome ( ꈍᴗꈍ)

It's something like surrogacy. The demon child was not connected to the mother by the umbilical cord. So was not getting life, nutrition from her at all. He was placed there by the curse/ devil. So like me the other comment said he was still technically born from the Mom. complicated, lol

They are still related to each other since they are half brother and sister. You said the devil impregnated the mother. Which would implied that they Half brother and sister that shares either a father or a mother to each other.

Technically these twins inherent half of the genes from their mother so half of the genes that are inside their bodies make them related to each other even if they are different species