I'm not certain but some of the .5 chapters seem to be "what if" or imaginary scenarios. One early .5 chapter showed the two kissing when they slept together, but the main story made it clear that never happened since the following chapters showed their relationship was still familial/platonic and the older brother didn't realize the younger held romantic feelings for him. Hell, they still haven't kissed even after becoming boyfriend and girlfriend; even a hug was embarrassing for them.