well said however your forgetting the psychological impact it had on the Shinya. it also alters his behavior. Yes it wasn't Yoshito's fault but he also allowed his brothers behavior towards his girlfriends. one of the girlfriend rules out due to falsifying he raped her.
However Ryuu didn't deserve anything that had happened to him. still he fell for Shinya not because of pity but because he wanted to make shinya feel loved due to his past. and his past is a big deal because it determines what kind of person you will be depending on the impact of the incident. His brother and father absence in parental control gave him the power over them, and the father used his past action as an excuse not to be part of shinya's life and dumped all the responsibility on Yoshito. Sad story with a happy ending.
This whole manga was pure B.S. why I say this for the younger brother o.k . you've had some bad things happen to you do to your father big deal that's life it happens .But you where not physically or sexually a bruised yeah you may have suffered emotional a bruise but that does not justify what he did to his brother girlfriends or friends .He's fucking nut's not only did he commit criminal acts such as rape he drugged people . He didn't even hurt the brother he hurt innocent people he should have been put away . But no he was let to do it over and over he hasn't learned any lessons. Instead he gets rewarded with a victim turned lover whom is just as stupid to even want to be near someone who brutely raped then give you a sob story and wham your in love for life .