Even if we throw away the age gap aspect, the chemistry between miki and that asshole is terrible.
Also, The parents only apologise only after Miki confessed. Did they not notice or worry when their kid come home late? Did they not ask why his grades plummeted? Yes I know that they focus on the 'gifted' younger twin but damn bro... Did they even see Miki as their son at all? Did they even ask themselves why Miki grew differently than Itsuki? I've read a lot of story about abandoned child, so the part where Miki's parents apologize threw me off the roof with how uncanny it feels. What's there to apologise in the first place anyway. They don't even want him, do they? Just rot in hell already.
I know it's bad since the first chapter yet I keep on reading the next chapter... It keeps on getting worse... Even with the glittery kira-kira effect, that last chapter is definitely a bad ending...