
Honestly, since this series is still going I think there’s another plot twist...
I have a feeling it really is his brother. It said he THOUGHT they might actually have a picture of his dead brother in the file folder. But the detectives never showed it. They made it very clear they wanted to discuss things with his family first. The body could simply be someone else too, just to throw people off and make them believe this twist and that it isn’t the brother. Also, the story the fake told could simply be made up.
I think the first chapter gives a lot of signs and more untold secrets. The elder brother in the car ride seemed genuinely pissed about being called gay. I think to some extent he was, or had feelings for his younger brother (indenial about being gay as well perhaps) Which is why he snapped after finding the video and seeing how his little brother mocked him with scorn.
I think he just really fucked up psychological issues, especially after getting his head hit. This is his opportunity to finally start over with his brother.
Anyway, that’s my somewhat dumb take.

Broken arm? there's nothing weird about that though unless I'm missing something. And the scene is visualized by the MC, not necessarily the "fake" since he's telling the OP it.
the biggest plot hole to my theory instead would be his fear and memory of being seated in a room when he goes to a therapist to recover his memories. That would imply he is a fake and not the brother.
I think there's more to it.

Ohhhh. I see what you mean now. But in the first chapter no panel explicitly states he even broke his arm. He uses his right arm, yes. And there's one tiny panel that alludes to his left having a dark spot on it (the clothing), but it's way too small to confirm. Also, his left arm is intentional covered in multiple panels by the OC in ch1, so the author know this just leaves things ambiguos.
Allow me to entertain you further with my dumb theory.
After his brother passed out, he came to and broke his own arm. The right one, the one he initially used to grab a rock and attack his brother with. The other one was fine all along.
Like imagine if the arthor pulled that on us.
welp, more rape according to the raws...