Sister: You're so stupid.
Me: Calling me stupid doesn't make you any smarter!
Sister: You're so stupid.
Me: Calling me stupid doesn't make you any smarter!
Alright listen up buttercup, here's the deal. The sister is calling the other person stupid. The joke is that the other person says that calling someone stupid doesn't make you smarter. It's like, duh, being mean doesn't make you cool, you absolute melon.