The fictional trope of the protagonist having an evil sibling is a time-honored tradition - and superhero siblings are no exception to the rule. With most caped crusaders having at least a half-century of published history behind them, most have experienced the old “secret and sinister sibling” reveal a time or two - even if a lot of those rogue relatives go on to be ignored by subsequent creative teams.
Whether an individual crimefighter plies their trade for DC or Marvel Comics, they’re likely to have at least one family member somewhere in their list of archenemies. Whether they share one parent or two, or whether they’re born at the same time or years apart, siblings have a tendency to move toward opposite moral poles when it comes to the world of comic books. Oftentimes, the draw of inserting some familial drama into a superheroic plot is just too strong for a writer to resist.
The existence of Wonder Woman’s twin brother, Jason - and the fact that they’re both the children of Queen Hippolyta and Zeus - wasn't revealed until a 2016 post-Rebirth storyline - but after that, the revelations started coming fast and furious. Jason and Diana initially get along just great - until he reveals that he’s in league with an individual known as Grail, the daughter of Darkseid.
Jason finds himself embroiled in a dark plot of Grail's that involves resurrecting her father, and that brings him into direct conflict with his sister. At the moment of truth, his better nature wins out and Jason prevents Grail from ending Wonder Woman’s life - though it’s yet to be firmly determined which side of the moral spectrum he’ll ultimately fall on.
Nobody does evil siblings like the Bat Family, and Barbara Gordon is no exception. Her brother - the oldest child of Jim Gordon, as featured in the iconic Batman: Year One - spends much of his life in psychiatric care, but he eventually gets out and embarks on a career as a serial slayer.
During the " Black Mirror" storyline, James Gordon Jr .’s misdeeds come to light - and he’s briefly brought to justice before escaping to terrorize Gotham City once again. At one point, Batgirl appears to slay her brother with a Batarang to the face, but he survives the experience - minus an eye. He later goes on to join Task Force X under the name of X-Strike - though there’s yet to be any hint that his motivations are anything but malevolent.
Jakarra is a result of an affair between King T’Chaka and a spy named Mateena - giving him a claim to the Wakandan throne despite the illegitimacy of his birth. He tries to demand exactly that when his older brother, T’Challa, ascends to the regency - and when he’s rebuffed, Jakarra doesn’t take it well.
T’Chaka’s second son makes several attempts against the Black Panther's life, and when that proves ineffective, he jabs some Vibranium into his own body in an effort to gain superpowers. Jakarra’s gambit succeeds, albeit at the horrific cost of transforming him into a purple scaly monster - and one that, somewhat embarrassingly, still can’t defeat T’Challa and his ally Ororo Munroe. Jakarra is tossed into a volcano and never heard from again.
Reed Richards’s father, Nathaniel, is a time-traveler - and that’s led to all sorts of familial complications for the adventurer better known as Mr. Fantastic. Nathaniel is typically an ally of the Fantastic Four, but he’s also thrown a wrench into their works on a couple of occasions, including the time he reveals that Doctor Doom’s adopted son is actually Reed’s half-brother.
Kristoff Vernard is - according to Nathaniel, at least - the result of a time-bending affair with a Latverian woman - though he ends up orphaned and adopted by the semi-benevolent Doctor Doom. Kristoff goes on to spend some time on the Latverian throne in Doom’s absence, and at one point, even believes himself to be the real Doom - though, overall, he’s usually been shown to be a lot friendlier than his dear old dad. There’s yet to be much in the way of fallout from the revelation, but it has managed to add another layer of complexity to the already fraught relationship between Doom and Richards.