Or he start to like seme as they entered teenage and didn't like uke interfere in them even that brother is shorter than seme so seme grow taller than him that's how uke witnessed them
well i think the crazy brother knew they had a thing between them and kissed him purposefully to break them apart he deaf new he was standing there, since he knew the mc was the only personnel his brother had as his own and cared for ,well since their mother was just practically forcing a fantasy on him and denning what was happening in reality
and he bullied mc , thats why she gave his dad hush money ig
I remember in first chapters the ML and his "brother" were kissing in high school and uke saw it... im sure that "brother "forced him he might have done crazy stuff to him.