Hi, I'll try ヾ(❀╹◡╹)ﾉ~
Seme has an older brother who has died. The older brother had a partner whom Seme will fall in love with. By time Seme and Uke's relationship become close and sexual. Seme who has feeling for Uke, started to feel insecure. He has an older brother who is loved and perfect in everything, he can't help but to resent it. As he witness how his mom try to make him a substitute of his death bro, he become aware of Uke's intention for himself. Is he a substitute for Uke's death lover? The breaking point is when Uke gave Seme a mantle that is so similar to previously worn by the older brother. This is answering why Seme behave that crazy in the last chapters.
There are some scenes that show Uke actually didn't love the older brother. It looks like their relationship is forced and only the older brother who is affectionate. Uke showed to be more interested to Seme, which the older brother noticed. However, this is still an invalid opinion. Maybe the next season will explain further.
Don't worry. You hit the nail. You're right.
can someone please explain to me the story in baby terms, I am so confused