I say it's her subtle way of challenging Kana. >8DDD They're like the warriors for Mihoko's heart....or something.... *hyperactive imagination* XD XD XD
I doubt Hisa has any idea Mihoko has a crush on her. The same could be said regarding Mihoko and Kana, but that may be because everyone on her team has affection for her.
Considering this is Hisa we are talking 'bout, she may be using a tactic she saw on TV to help Kana admit her love for Mihoko: using something cold to make her relax or something, y'know?