In this story, Tatsuya doesn't know when he starts a relationship with Haru that he's his younger brother. When he finds out, he tries to stop all intimate actions with Haru despite the feelings he has let develop. When he finds out Haru may not be his brother (by blood) after all, he takes that opportunity to accept his feelings because he has already stopped seeing Haru as his brother (remember, he only knew Haru as a little kid - at most Haru was 5/6 years old since Tatsuya was wearing his middle school uniform and he's 9 years older. So, it's not that hard to imagine that Tatsuya has trouble looking at the older man, Haru, and seeing that little brother of his when he'd already fallen in love with Haru). Tatsuya mentions at that point that it no longer matters to him whether they are related by blood or not - which he later finds out they are.

In Haru's case, he presumably was told at the age of 5/6 or younger that Tatsuya was not his real brother, therefore, he grew up thinking that way. Up until they meet again, he holds only the utmost admiration for Tatsuya. When Tatsuya seduces him, Haru gives in to it allowing his feelings of admiration to change to those of love. In this case, although he knew Tatsuya was the brother he'd once had, they had not grown up together and instead he grew up believing they were never real brothers in the first place. It's not hard to imagine that Haru could let go of that feeling of 'brother' at this point having lived for more than two thirds of his life without Tatsuya by his side.

I'm glad that it had a happy ending.

Wow, it really was incest