Underdeveloped? Maybe, if you're blind. He was shown the whole series to have very, VERY much interest in her and her happiness. "Hey, I'm gonna hook up with a girl I'm not really interested in, so the guy my sister took a fancy to will go out with her." They had even a hint or two that it goes beyond brotherly love, like the helmet fastening scene. What do you need to call it developed? 3 episodes focused on him huffing and puffing alone in a dark room crying out her name and then asking himself if it's wrong to sniff his little sisters panties? TT has many flaws, but at least in this case, it's just you being dense.