The word (huge) violates rule #1 and #2. We are not talking about a tall, slender teenager. Dekai implies bulk: wide shoulders, thick chest, heavy arms. In fan art terms, we are describing a bear or bodybuilder archetype—the kind of character you expect in Bara (geikomi) manga, not in a typical "cute little brother" story.
Riku laughed, a booming sound that shook the walls. "You're the worst, Sis."
The humor comes from the mismatch between the detailed setup and the nonsensical punchline.
But the biggest source of heat wasn't the sun. It was Riku.