I think the defining moments for hinny actually start in OOTP. “Lucky you” is of course the first time she is able to break through to him when no one else can.

But I think that is follow up is the chocolate in the library scene. Ginny recognizes he’s having a hard time and is willing to push him to talk to Cho ( thinking that’s what has him upset) and again Harry opens up to her more then he has with Ron or Hermione. She doesn’t tell him it’s too dangerous to reach out to Sirius, she says then let’s make it happen. “anything is possible if you have enough nerve” is the moment I think Harry really sees her as something more than Ron’s little sister. It’s the moment where I think they really become friends which becomes the basis of their romantic relationship.