“Kagenui, no brother would let out his little sister’s embarrassing secret [that she’s not human]. I don’t even care about checking on something like that.”
To put it in words that Hachikuji Mayoi might use, this is the courage to continue keeping a secret.
“Because it’s for family, I can lie. I deceive them. I make trouble for them. I annoy them. I can borrow something from them and not return the favor. But still, I think that’s okay.”
- Nisemonogatari Volume 2 “Tsukihi Phoenix” Chapter 12 (emphasis mine)