@Darudius
Wait his sister asked about the matching pair of dog keychains. Did he buy 2 last chapter? I thought he bought one? Did he give one to his sister and the other to Yamada?
It wasn't clear to me until this chapter, but it seems that when his sister was teasing Kyoutarou about getting a matching pair, she meant one for him and one for her. When they're checking out, he says he'll pay for his own, so I think he only bought one. Presumably, his sister bought her own.
From what she says at the end of this chapter, it seems that his sister thought he was going along with her idea of having a matching pair and is unaware that he got it as a gift for Yamada. I assume this will come up when they inevitably meet each other at the upcoming shrine visit.