It was something like them finding comfort in each other and having sex each night but then Yukio starts getting more obsessed with the sex(not sure if this is true or not but its what the twin said) and the twin becomes afraid of him. so one night seeing Yukio masturbate the twin drowns Yukio by using the fish tank and throws yukio's body in the ocean/river. Yukio's body is found and a funeral is held. Afterwards the twin is haunted by the death of Yukio and imagines Yukio coming back to get his revenge. Which is why the twin almost dies, he tried to drown himself with the fish tank too. Basically the ending is just the twin going crazy. Hope this helps but I couldn't explain well
What the fuck happened to yukio? At first i thought he cut his dick off and died while doing so but at the end it felt like he was killed by his brother or even more like there was no yukio to begin with and he just made his twin up - so if anyone understood chapter 3, pls enlighten me