Sisters is the second episode of the series and season one.
Blackfire.
Synopsis
The Teen Titans are enjoying a fair when suddenly Starfire is attacked by an alien probe. The team succeed in fighting it off, but are left wondering where it came from. They return to Titans Tower, where they find that Starfire's sister Blackfire has stopped by Earth for a visit.
Starfire is overjoyed to see her at first, but Blackfire quickly begins taking over Starfire's place on the team. They visit a disco, where Starfire is left alone when the other Titans manage to find partners for a dance. Robin tries to talk to her, but Blackfire drags him off. Starfire once again encounter the probes, which manage to overwhelm her. The Titans come to her rescue, where Blackfire destroys all the probes with starbolts from her eyes. Cyborg asks her how she did it, where she replies with a hint of smile that it was just a lucky guess. Robin remarks that they need that kind of luck and offers Blackfire a spot on the team! Starfire, who is still recovering from the attack, gasps in shock.
Starfire is sad about this at first, but then comes to the decision that the team will be better off without her, and decides to leave. Robin convinces her to stay, but just then, the aliens who sent the probes show up and take Starfire captive. Robin attacks their ship and sets her free, but just as the other Titans show up and prepare to battle, the aliens announce that they are Centauri police, here to capture a Tamaranean thief. Robin realizes they must be talking about Blackfire, who they then notice is attempting to escape the planet. Starfire, enraged, flies after her sister, and engages her in combat long enough for the Centaurians to capture her. Blackfire is taken away by the Centauri Police, vowing revenge on her sister.