there’s definitely something to that. i personally think that Mitsue hated Ranzou simply because he was the result of an incestuous relationship between her husband and his sister. she couldn’t target her resentment towards her husband, as she was still kind of dependent on him, but also as you said, she desperately needed attention (and i also think she was really well manipulated by Seizou, which can be seen in Norihiko’s flashback at the very beginning of chapter 20 - Seizou told her he loved her and she also said she loved him; and it was when Norihiko was a little older, so after Ikurou was born and after Mitsue found out about Ranzou). she couldn’t quite bite the hand that was feeding her, so she treated Ranzou as the source of all the problems.
i think that there’s some possibility that Hanae was a victim of sexual abuse by her brother. it’s important to keep in mind that what we can see at the end of chapter 23 is Mitsue’s memories, so she could’ve twisted Hanae’s facial expression, the tone of her confession or even the exact words of it, simply because she was so disgusted and enraged. human brain can change the memories without the person being even aware of it. also, i didn’t include it in the translation notes, but when Mitsue says “What do you think Hanae said?”, there’s actually furigana (which indicates specific reading of the characters) beside kanji characters for “Hanae”, which wasn’t present previously when her name was mentioned. the furigana reads “あの女” which translates literally to “that woman”, but it’s rather direct and considered rude to address anyone like that. so it exemplifies even further how enraged Mitsue was and that her brain could’ve changed (or interpreted differently) some of her memories. but i think it’s also possible that Hanae could’ve really seduced her brother. the thing is, there’s not too many hints about what their relationship really was.
i think Mitsue blamed Seizou but due to how misogynistic and sexist the way of thinking was back then, she probably knew that she shouldn’t focus her hatred on a man, but on a woman/child instead.
We knew a lot about the family's backstory through Ikurou's perspective, so now giving Ranzou's perspective opens things up a bit more. Here's what I'm thinking:
A long time ago when a woman was sexually assaulted she was blamed for "seducing" the man. I wonder, did Hanae really seduce her brother or was she led to believe that? Also up until many years ago, children would sometimes also be blamed if they were molested, saying the same that they "seduced" the adult. If the abuser grooms them to believe that, they can develop Stockholm syndrome. I wonder if Ranzou was like that cuz he seemed to grieve their father when he died. Ikurou certainly developed that for that bastard Norihiko.
Now with all that, I'm starting to believe Ikurou's mom Mitsue didn't try to kill Ranzou out of jealously or because he "seduced" his father (cuz it didn't happen yet), but because she couldn't live with such a tainted child. She wouldn't target her husband because A) she was starving for attention and approval and B) she blamed the woman, Hanae told her, thus this spawn of evil should be purged from the world, but the man not punished. Sadly, after she let Touma keep Ranzou alive, he started molesting him - he likely was already a rapist but he wanted to take advantage of the fact Ranzou couldn't speak due to being now mentally retarded. Mitsue already saw him as the spawn of evil but now he was "seducing" her husband, so she started calling Ranzou filthy, carnivorous, a mantis. But over time she knew how disgusting her husband was. Because of that it was easy for her to fall for Norihiko. He was someone who paid attention to her and was seemingly normal (big mistake, he turned out to be just like her husband, she sure knows how to pick em).
Mitsue was already mentally disturbed but living in this incestuous family really drove her over the edge. I wonder if the same happened to Hanae. It's not her fault but it's ironic how Mitsue's actions to escape that insane family by running into Norihiko's arms would end with HER son ending up in the same position as Ranzou: goomed, manipulated, trapped, isolated, abused, convinced they were the seducter, called the mantis. Both brothers, regardless of their status at birth, became victimized by the evil men of this family (Norihiko counts cuz his family has served the Toumas for a long time).
That was a lot, so what do you guys think? This is sooo insane