Not sure if this will be in the story or not, because it's both complicated and awkward to fit in so I have not been able to do it yet.
There are two brothers in the story, Osiris and Set, based off the Egyptian gods of the same name. The two do not get along, they just can't cooperate, are always arguing or fighting. The disagreements getting so bad that at one point Set murders Osiris (but Osiris is revived because gods and mythology said so)
Because this event happened before the comic starts and is more character history it's not shown in the comics, and because neither Set nor Osiris are willing to talk about the event it never really comes up to the point where either will get into detail as to WHY the fighting got so bad that one killed the other.
Though the fighting and some reasons why these two do not get along have been brought up in the comics the actual scenario in which Set killed Osiris is never really explained in the comics, those who know mythology and the gods these two characters are based on though may have a better idea.
The reason is basically Anubis's conception, (who is dead in the comics along with most gods in this line) though he is regarded as Set's son he is actually Osiris's son. This happening through the plotting of their wives after Set refuses time and time again to provide his wife with a child (he's gay and has no interest in having children with his wife), so to fix this situation, his wife (with the help of Osiris's wife) disguised herself as Osiris's wife to sleep with him and get herself pregnant with child.
Osiris is kind of an idiot, so he's completely unaware of the trickery that went into play and fully believes that Anubis is the son of his brother, Set is not as stupid, and knows exactly what has gone down, and as a result kills Osiris out of jealousy for the things that he has taken from him.
It's just an awkward thing to actually explain in the story, the characters KNOW the events that happened, in canon, these events HAVE happened, they even make small references to it, but because it happened in the characters pasts, before the story starts, it never actually gets explained because I'm not sure where to naturally throw this information in, right now, only readers that know about the Set Osiris mythology may be aware of the issue, and only if they've read that version of the myth (as there are several version of the Set Osiris myth)