Andor satisfyingly tied up many loose ends by its season 1 close, but the Star Wars show had one central mystery that it didn't resolve. The opening moments of episode 1, "Kassa," featured Cassian asking questions in a brothel on the industrial planet Morlana One, before accidentally killing two officers who accosted him. Cassian fled to Ferrix to hide, setting in motion the show's events that led him across the galaxy while growing a conviction in the Rebellion cause. However, Cassian's purpose for visiting the Morlana One brothel, to discover a lead on his sister's whereabouts, was not revisited by Andor season 1.
Cassian's sister, Kerri, was seen during flashbacks in Star Wars' Andor, back on Cassian's homeworld of Kenari. After a Republic ship crashed nearby, a group of older Kenari younglings left their village to investigate, with Cassian joining the expedition and leaving Kerri. As he was taken away from Kenari by Maarva in a flashback in episode 3, "Reckoning," that farewell was the last time Cassian saw his sister. It was also where Andor left her character, preserving the mystery of what became of her.
Andor Hasn't Confirmed What Happened To Cassian's Sister
Given how committed Cassian was to finding his sister in "Kassa," he clearly felt guilty for leaving her on Kenari. The Republic was on its way to investigate its crashed ship and might have apportioned some of the blame for the dead bodies around the ship on the natives, and taken revenge upon them. Kerri could have been killed then, or at another stage during Kenari's mining disasters under both Andor's Empire and the Republic. This was a pessimistic suspicion Cassian harbored in Andor, despite his gratitude towards Maarva for giving him a safe home.
However, he also desperately wanted to believe that she was both alive, and managed to escape Kenari. The reason he visited the Morlana One brothel was that he heard a Kenari girl worked there, before being told she left months earlier. He was grasping onto any hope, no matter how fickle. The last time she was mentioned in Andor season 1 was during Cassian's final moments with Maarva after the Aldhani heist, when Maarva told him to stop searching for his sister, and that he shouldn't blame himself for what likely happened to her.
Could Andor's Sister Appear In Season 2?
Given Andor's reputation as adult Star Wars based in mature storytelling, it is entirely possible that Kerri does not appear again. Cassian's guilt over losing her was the catalyst for his arc in joining Luthen's rebellion in the season 1 finale, and that is potentially all she might represent for both seasons. If Cassian were to meet Kerri again, it would be the kind of dramatic coincidence familiar from the Skywalker Saga, but alien to Andor's more grounded sensibilities. Maarva might have been correct when she told Cassian that everyone on Kenari died. Likewise, if Kerri did escape, she was somewhere among the universe's thousands of planets and billions of life forms.
However, it is also possible that she might show up in Andor season 2. While it might be a slight digression from Andor's more realistic tone, reuniting Cassian with Kerri would make narrative sense for the Star Wars show, as a way to connect the Republic's past colonial crimes with the Empire's contemporary cruelty. Her absence from Rogue One: A Star Wars Story will have to be explained too, but the show could turn that on its head. If Cassian finds Kerri only for her to be tragically taken away from him again, it could explain why he was so jaded by the time of Rogue One.