I remember as a kid, reading a Star Trek reference book--probably a later edition of the Star Trek Concordance--with a footnote, to the effect of the writers had decided that the main crew, for their purposes, were all only children. I'm not sure if that was by editorial fiat or democratic decision; but they didn't want anyone pulling out that hackneyed old soap-opera plot of Spock's sister or Kirk's brother showing up--or worse, having to cast a guest-star as such. And they broke that rule in Star Trek V with Spock's half-brother, and look how that turned out.
You could um, actually that quite a bit: given the nigh-irresistible seven-year mating cycle of the Vulcan's pon farr, it would be weird if Spock didn't have any siblings. And Kirk's brother Sam appears in the episode Operation--Annihilate! but is killed before we see him. And Scott had a nephew in Wrath of Khan, his sister's son...anyone who makes any of these points is technically correct but a huge nerd and you have my permission to punch them in the arm as hard as you can. The larger point remains, generally it's better to avoid bringing in previously unseen and unmentioned siblings to generate a story.
Anyway, today we meet Conan's sister...oh, goddamnit...
From 1985, Savage Sword of Conan #119, "Homecoming" Written by Don Kraar, art by Ernie Chan. These stories were usually continuity-light, so Conan could be a pirate on the Vilayet sea one issue, and in his homeland of Cimmeria the next--for those of you who don't know your Hyborian Age, those locations are not close together. But this one introduces Conan's sister, Siobhan; and a different fate for Conan's parents than was used before--or since--killed by a rival clan, the Diarmiads. After saving Siobhan from a wild boar, Conan learns his parents' fate, and vows vengeance. Siobhan urges caution, and the Diarmiads can't just kill Conan outright, since it was the Feast of Crom, and forbidden for Cimmerian to kill Cimmerian on that day. (That doesn't really hold water: Conan and his people were uncomplicated barbarians, far too practical to hold to such a rule; and their god Crom was often described as utterly uncaring as to what happened to them.)
Conan introduces himself as "Amra," an old alias; but he knows he's not fooling anyone. During the feast, the Diarmiads toy with him by threatening Siobhan, and Conan is challenged to throw axes at her head. Which he does. Blindfolded. Maglocun, the chieftain, invites Conan to a hunt in his honor the next day, calling him out.
Siobhan tells Conan that Maglocun is charmed, and only their father's sword could kill him, but the Diarmiads had it beat into a plowshare. At the hunt, Conan is given a lame old nag and a nub of a sword, but Conan still remembers the lay of the land from his childhood, and turns the tables on the hunters. A dog manages to knock Conan off a cliff, but the Diarmiads aren't about to assume he's dead. Injured, Conan makes his way to his father's forge, and the next day two of his enslaved cousins deliver the plowshare, for Conan to reforge back into a sword. That night, the slaves turn on their captors, as Conan faces down Maglocun and his immediate family. They try to make it seem like Conan is outnumbered six to one, but c'mon, it's Conan: he probably could've killed every last one of them himself. I've joked before that Conan had killed more people than cancer at that point, but I don't think people got cancer back then. They died of Conan first.
Conan halves the simpering Maglocun with his father's sword: it's not clear if Maglocun was actually enchanted, or if Conan even believes it. He and the other Cimmerians are sometimes portrayed as superstitious. But he's practical, and even if the symbolism didn't appeal to him, Conan may simply figure it's just as easy to kill him that way. Siobhan dies shortly after the battle, having been whipped pretty soundly then getting stabbed while killing the Diarmiad's witch, Morag. Neither she nor the sword of Conan's dad are ever mentioned again, possibly because Conan isn't the sentimental type, or because this may or may not fit with your personal continuity for his stories. For me?...maybe.