He is completely and utterly in love with her. The only problem is that, since he was a child, he has been trying to act like "a responsible and mature adult", just like the newsreader who appeared in chapter 59.

Essentially, his sense of identity revolves around his sense of self control and maturity. Mostly because she is younger than him, and technically not an adult yet, he believes that it would be selfish and childish if he were to make a move on her, and therefore he believes that his feelings for her contradict his sense of identity. This is highlighted in chapter 59 when he says something like "my ability to control my emotions is the only way I can stay sane".

Shiharu is also in love with Matsunaga, and not because he is a "responsible and mature adult", as he tries to pretend to be, but because he is actually a good person. This is an important point when you consider that almost everyone else in the manga can only see the "mature and responsible" matsunaga (aka, his image on TV).

She has some extreme complexes about being left alone, and not having a 'home' or a place that she 'belongs' (although this particular aspect of her behaviour is irrational for various reasons that I will not bother explaining). It should be very obvious that she relies on the 'family' that they have created.

The interesting thing is that Matsunaga relies just as much on this 'family' as she does. Therefore, when that 'family' disappears (when the twins leave), he is going to be just as affected as she is. One of them will invariably "give in", and they will invariably end up reconciling the feelings that they have for each other.