And that gives the MC freedom to cheat on Akiho? This is before he even knew, you're trying to excuse the MC for his wrongdoing just because Akiho did it first.
About the no remorse no apology, the MC can't even refute or say anything back after his initial shock, he even admits himself that he's a coward, because he knows he also did wrong. Despite this revelation he knows he have affection for akiho and willingly cheated on her.
Your informed consent argument falls short, because when you take away this withheld information. He has asserted many times that he still loves fuyu and will take her back regardless and yet still decided to take Akiho as his girlfriend. I don't know why you're even trying to argue that people shouldn't have a mature conversation and just leave this problem hanging, just because she's manipulative. Confront her and settle if he wants to break up or not after all, a sister is a sister, and a girlfriend is a person that he has feelings for.
So the MC gets blamed for being confused, but Akiho gets off easy for not telling him she was his half-sister and still starting a romantic relationship? That’s not just messed up, that’s some next-level bullshit.
This isn’t about who screwed up first. It’s about how big the screw-up was and what they were thinking. The MC was torn between two people he cared about, that’s messy, but it’s not malicious. Akiho? She knew the truth and chose to hide it because it would ruin her chances. That’s not just lying, that’s manipulation.
And no, the informed consent argument doesn’t fall short. It’s the whole point. She robbed him of the chance to make a real choice, not just emotionally, but morally. Trying to compare that to him having leftover feelings for someone else? That’s asinine.
And now you expect the MC to have a "mature conversation" with her? Give me a break. Akiho was calculated enough to play this whole game, but now the MC is immature for not sitting down and handing her emotional closure after being kept in the dark about something this big? She made her choices. He doesn’t owe her anything, not his time, not his forgiveness, nothing.