So...

Yoonjae acts like a child as a defense mechanism he created after being abandoned by everyone as a child, by acting like a helpless child the big "brother" would leave whatever he was doing and comfort him. He got used to act like child whenever he wanted something and his now boyfriend never corrected him. This is also a way to evade real life, he has a hikikomori lifestyle and doesn't socialize at all by his own decision, he only wants his boyfriend/"brother" in his life, who now is forcing Yoonjae to socialize with Wondae and Yoonwon since he doesn't even like talking to his brother, he even hired Wondae to spend time with Yoonjae. It's just trauma, nothing to hate over here, as weird as you can find it you shouldn't hate a traumatized person, especially when their partner is trying to find a way to help them heal.
About our couple, Yoonwon apologised for the wrong thing, he should be begging forgiveness for raping Wondae in the first place, the was his biggest mistake not saying shit to him