Sina is a bit surprised, and embarrassed to boot, when Nari rats on her. She would never use her sweetness to get ahead in life!
...definitely not. And that's why she's blushing and looking quite guilty indeed.

"It didn't work that well," she mumbles into Lux's fur, turning the both of them away slightly so she can be properly evasive. "And no more parties for me, thank you. They're too noisy and crowded."
This is said with a near-primness, a tone Nari has no doubt heard before: the Miracle Baby's response to being picked on. She's above it, don't you see.