In honor of Cass’ new brother, I thought I’d make a list of some of Cass’ other siblings on her dad’s side
1. Mad Dog
Cain’s first experiment is his adopted son Mad Dog. Cain drove him crazy with his abuse and decided to discard him. One of the guards spared Mad Dog but set him into the wild and basically raised him as an outdoor pet until Nyssa al Ghul convinced Mad Dog to join her League. Cass and Mad Dog have a fight, Cass ends up knocking him out but gets stabbed and dies in the processes and Shiva has to stick her in a Lazarus Pit. What happens to Mad Dog after Cass defeats him is not shown.
Batgirl Vol 1 #71
2. Annalea
Annalea is an assassin for hire also trained by Cain. She’s gets introduced by seducing and killing a man, before Cassandra beats her up and kills her. Apparently Cass is so jealous that Cain loved someone besides her she murders her sister and then tries to force Tim to kill Cain. Thankfully it’s later retconned that Cass turned evil because of brainwashing instead of jealousy, but Annalea’s still dead lol.
Robin Vol 4 #149
3. Marque and the Cain/Deathstroke expiraments
During Batgirl Vol 2 we meet Cass’ older bio sister, Marque. Ever since escaping Cain, Marque has dedicated her life to hunting him down and killing him.
Batgirl Vol 2 #1
Speaking of the evil!Cass arc, Cass meets her while she’s also hunting down Cain, as well as Deathstroke, to get revenge on them for brainwashing her into becoming a killer. Cass learns that Cain has been raising even more girls and also giving them Slade’s super strength serum, training them to take down the meta-hero community.
Batgirl Vol 2 #3
Marque was hunting down these girls and murdering them, giving herself a scar for every “hint of Cain’s blood” she wiped out. Between Marque’s killings and Slade deciding to cut his losses and blow up their base, a majority of them died, but Cain does say there are still some left.
Batgirl Vol 2 #4
Anyways, Cass stops Marque from killing Cain by beating her up in an alley and seriously injuring her, which makes Marque vow that Cassandra is her enemy, not her sister.
And then the dc universe gets rebooted and Cain’s backstory becomes completely different so who knows what siblings Cass has on her father’s side these days 🤷