The other Kara Kent on TV was from Superman: The Animated Series. In that show, Supergirl was Kara In-Ze, so she wasn't Kal-El's cousin. But her secret identity was Kara Kent, Clark's (supposed) cousin.
Kevin might be more familiar with her than with Kara Kent from Smallville. Or maybe he's just REALLY familiar with the comics (as there was plenty of those with Kara Kent/Kara Zor-El). Either way, he suspects that she thinks of herself as related (directly) to Clark Kent/Superman (whether as a cousin or the child of).
CLARK: No. I'm just worried I'm a jinx.
JONATHAN: A jinx?
CLARK: Yeah. Let's face it, ever since she's known me, Lois's been kidnapped, frozen, pushed off buildings, almost stabbed, poisoned, buried alive and who knows what else, and it's all because of me.
-"Contact" (You're not her jinx, you're her blessing.)
| 
