No, he's not, Ranzo is the older brother, Norihiko is twisted from just sleeping with the Ikuro's insane mother at a young age, he was in High School.
Yeah... But if you read the mother interaction with Norihiko she says" do not go to visit the woman in the douzo". I mean she is quite persistant that Norihiko should not visit "that woman" but be with Ikurou. Also Ikurou says in one panel "Ranzou should have been left in the douzo". It is just the way the mother is talking to Norihiko it seems like he is connected to Ranzo somehow. Also he does not manipulate Ranzo. His sympathy to Ranzo is bit different than how he sympathizes Ikurou. And also in few panels he is seen overseeing Ranzo and his caretaker. I could be wrong but i feel somewhere something is connected.
that's because she's insane and she talks to norihiko as if he was her husband. that's why she's asking to not see that woman anymore. ranzou is not her son
it's a bastard that his husband took at home
What she/he said about is correct, the madam, was insane and used Norihiko as her husband, hence, why he caught her off guard when he kissed her, I guess her husband no longer kissed her and it was out character for her husband, and so in that moment she recovers.
Hmm... i am rereading it again. I may be wrong here but could ranzou be norihiko's son