There's a long-established trope in media of family who married in, were taken in, or some other variation being mean to their new siblings. Everyone knows the story of Cinderella and the mean step-sisters but there are tons of examples seen in movies, anime, and video games as well.
Ace from One Piece, for example, is awful to Luffy for an incredibly long time before coming around to him. Meanwhile, Thor and Loki's relationship in the Marvel movies (and obviously the Norse Mythology source material) is anything but healthy. So, let's take a look at some of the worst stepbrothers in video games specifically.
7 Dr. Hal "Otacon" Emmerich: Metal Gear Solid Series
Starting off with probably the most well-known video game stepsibling, Octacon from both the retro and more recent Metal Gear Solid games. Of course, this is only introduced as part of his character in the bombastic sequel MGS2: Sons of Liberty, but from that point on it really paints this loveable nerdy coward in a much more sinister light. The implication behind Hal's relationship with Emma's mother (his stepmother) is that Hal was the victim, but it's never confirmed. And, it doesn't change the fact that he didn't hear Emma drowning due to 'being with' her mother or that he left home, abandoning Emma shortly after.
In the grand scheme of things, Hal made what he thought was the best decision at the time and had good intentions overall, but it didn't change the fact that Emma spent most of her life hating Hal and trying to be the opposite of him out of spite.
6 Lee Chaolan: Tekken Series
Now, this next stepbrother is one that, since Tekken 4, has had a change of heart, but Lee Chaolan was originally a pretty awful stepbrother to Kazuya Mishima. To be fair to him, Heihachi literally took Lee in to raise him as Kazuya's rival, and Kazuya turned out to be pure evil, so he only did what was expected of him. Still, the two Mishima prodigies were constantly at each other's throats or trying to kill each other. Then, after siding with Kazuya during the King of Iron Fist Tournament 2, Lee was banished from the Mishima Zaibatsu and settled down in a mansion on his own until the events of Tekken 4.
After this, Lee constantly reappears as a fighter, but not as the devilish wannabe Mishima Golden Child, but rather as the CEO of Violet Systems, an eccentric and charismatic man who just loves the word 'excellent'. And, with Tekken 8 in development, who knows how Lee Chaolan will look in the newest game, considering all the differences fans have already seen in Nina Williams between Tekken 7 and Tekken 8.
5 Asirel & Flowey: Undertale
Big spoilers for Undertale ahead, but it did come out in 2015, so that's a long enough courtesy period. In any case, Asriel Dreemurr is the biological son of Toriel and Asgore, the king and queen of the monsters in the Underground of Undertale. Now, as a stepsibling, Asriel was actually a fantastic brother to Chara (the first human to fall into the Underground).
But, after both siblings passed away and Asriel was brought back (unintentionally) as Flowey, he becomes pretty twisted. In fact, despite how similar the protagonist of Undertale looks to his original sibling, Flowey does some truly awful things to them and is all around about as twisted as they come with their grandiose scheme of world destruction.
4 Travis Touchdown: No More Heroes Series
Technically, this next character is a bad stepbrother to multiple siblings, and it's not someone many people would guess. Travis Touchdown is the protagonist of the No More Heroes games, and that title is right because Travis is absolutely not a hero. He's an assassin that, for most of his early appearances, only kills to feel badass and prove he's the best fighter alive.
Now Travis has two stepsiblings, one is Jeane, a character who only ever appears briefly at the end of NMH1 (who Travis 'dated' when they were younger) to have her backstory literally fast-forwarded over, and the other is Sylvia who was technically his sister-in-law but later became his wife.
So not only did Travis unknowingly date one stepsibling, he then married his ex-sister-in-law after their divorce, making his actual brother Henry hate him even more.
3 The Husband: Twelve Minutes
While Twelve Minutes is an interesting game in terms of its setting, the voice talents behind the cast, and the overall look of the game, the story is not one of the aspects commonly praised. And, quite frankly, this is because it's nonsensical and confusing.
To explain it as simply as possible, the player character is a horrible stepbrother in this game because he literally marries his stepsister and murders their father (who was abusive, to be fair) without her knowing. Of course, a lot of this comes from ignorance, as neither sibling is aware of the fact that they're partially related, but considering one of the endings has the couple stay together in the end, it's still pretty messed up.
2 Kratos: God of War Series
This next one is half-loophole, half-legitimate, as Kratos is technically a character not in Greek mythology, but every other character around him is. Now, considering that Zeus is his father, and Kratos kills all the Greek Gods by the end of God of War 3, it should be pretty obvious how he's a bad stepsibling.
Granted, the Greek Gods in these games are scumbags themselves, and Ares, in particular, was a horrible stepbrother to Kratos too, but murder is still murder. For those who don't know off the top of their head, with Zeus as his father and Hera as his step-mother, Kratos is technically a stepbrother to Athena, Ares, Persephone, Hercules, Artemis, Apollo, Hermes, Perseus, Minos, and Pollux, which are almost all names of people Kratos has killed at some point or another.
1 Most Of Marika's Children: Elden Ring
And finally, let's talk about Elden Ring. The family situation of the lords in Elden Ring is also pretty comparable to the Greek Gods, as their family tree is absurdly difficult to parse. But, there are three 'families' to keep in mind: GodfreyXMarika, MerikaXRadagon, and RadagonXRennala. There were eight children that came from these three partnerships, and all of them are stepsiblings to many of the other children in some way.
For example, Godwyn, Morgott, and Mogh are stepbrothers to Miquella, Malenia, with Mogh's treatment of Miquella being obviously twisted. Not only that, but Miquella and Malenia are stepsiblings to Radahn, Rykard, and Ranni, with Malenia basically killing Radahn with Rot.
And finally, once players start considering the fact that Marika and Radagon are the same 'entity' (something that's discovered right near the end of the game), then that makes all the children partially related, which makes Ranni's murder of Godwyn the crime of a stepsibling as well. There's no specific character to use as the example here, all of them are awful stepsiblings to each other.