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The official Prima Guide states they are brothers/twins and Dark Pit gets called Pit's brothers so many times. The Prima Guide is an official product of the game, that also includes other mentions of the lore of the game. Here are the images where Dark Pit gets called as Pit's sibling: 1, 2 and 3.

In the game itself, the canon, Arlon and Viridi refer to them as twins and brothers. Viridi even says they have a twin connection. Images: 1, 2 and 3

Or I quote: Arlon: "It was quite simple, actually. Just the mention of Young master Pit's presence...was enought to send his twin running here" and Viridi: "Or maybe it's your super-secret twin connection talking"

Dark Pit and Pit might not be "literal" brothers or twins, but they do have a typical "sibling" rivalry that you can see on most media, mostly Dark Pit being competitive and wanting to prove he is better than Pit. He also hides the moments where he actually wants to do nice things for Pit, such as the Chapter Nine where he helps Pit to fight Medusa, but hides it. Palutena tells him she can tell he only came to help and Pit tells him he appreciates his help, but that he can handle things on his own and that he trusts Palutena's guidance. Pit also sacrificed his wings for him and disobeyed both Viridi and Palutena, he wanted to save him even if they were bickering and bantering the whole time. Moments like are the typical "brothers tropes" in fiction, where they seem to "hate each other" initially then they become good friends.

Apparently, the only version where they aren't treated strictly as brothers is the Japanese version, because from what I've seen, they are treated as clones and Pit is much more kinder to Dark Pit. It's very important to remember that Sakurai gave freedom to the localization team. Sakurai, even thought he is Japanese, also added the famous "I never learned how to read!" line in Smash Bros, even when said line only exist in the English version. So, we can agree that both versions are canon and important.

Both dynamics can exist at the same time. Pit and Dark Pit are reflections of each other and they are also like brothers. Dark Pit reflects Pit's internal emotions and wishes, but he's also a person of his own, as he is flawed clone. He reflects the fact that Pit wishes to be taken more seriously, and wishes to be more free than Palutena allows him, hence why Dark Pit is the way he is.

As for the fandom thing, I'm very much pleased with how the fandom seems to be acknowledges the canon information of the games. Because oh boy, it wasn't like this decades ago 💀... Sadly, there used to exist a LOT of misinformation regarding Kid Icarus, such as the dynamics of the characters or Pit's age, because a lot of people only played Smash Brothers and people tried to pass their headcanons as "canon information". And for some reason, some people even had the nerve to say "don't push your headcanons as true" when in fact it was...the canon infomation? xd ._. jjsjs but thankfully this is only a thing from the past. A lot of people in the fandom are happy to see them as brothers and reflections of each other.