Here are some witty phrases you could use to playfully tease a brother who's being silly or not thinking things through:

"You're not stupid. You just have bad luck when thinking."

"If brains were dynamite, you wouldn't have enough to blow your nose."

"I'd agree with you, but then we'd both be wrong."

"You're about as sharp as a marble."

"The wheel is spinning, but the hamster is dead."

"You're proof that evolution can go in reverse."

"If ignorance is bliss, you must be the happiest person alive."

"I'm not saying you're dumb, but you've got more gaps in your knowledge than Swiss cheese."

"Your IQ test results came back negative."

"You're so dense, light bends around you."

Remember to keep things lighthearted and good-natured when teasing siblings. The goal should be to share a laugh together, not to actually insult or hurt feelings. It's best to avoid truly mean-spirited comments and instead focus on clever wordplay or absurd exaggerations that you can both find humor in.