Naturally Starfire would be extremely jealous due to her insecurity when being compared to her sister. Blackfire is ٹھنڈے, کولر and prettier than Starfire. Starfire has the school-girl crush on Robin, so she would be crushed if Robin dumped سٹار, ستارہ for Blackfire. Robin and Blackfire would be a cute couple!!! :D
She would probably cry in her room because she already thinkgs that Blackfire is better than her in a lot of ways, but stealing Robin from her?! she would be as sad as Echo in the story of Echo and narcissus
Well it could be a possibility but I don't think she would kill her sister cuz remember in sisters even when BlackFire was being mean Starfire still tried 2 b nice 2 her.