They're half-siblings
I'm going to say "no.". The reason being that an editor's note started this story off by explaining that "family" in this sense is about yakuza groups. So for Yutaka to be the MCs "half-brother," would be to say the MC and Yutaka knew each other from the same "family" until leadership roles changed.
On the other hand, we all know Japan isn't above throwing in some actual incest in their stories...
On page 12 the chairman refers to them as half-siblings and we see them interact as teenagers. The implications are pretty clear I think
Not really. The both of them were still part of a yakuza family. I'm simply casting doubt due to the fact that Japan is notorious for casting figurative family ties and there's still room for them to not be blood-related, especially since the MC was cut-off from correcting what the guy meant when he called them "half-siblings."
As I said, they may or may not be. There might be an explanation later on or their relationship really is as degenerate as seen.
Are they related I though he was saying aniki because of gang stuff ☹