It’s heavily implied because in chapter 5 it says he was probably more like a “real son” to him than the uncles actual blood related kids. It’s also why he didn’t get a dime when uncle died, he’s not part of the actual family at all. This is how I interpreted it, idk how else you can interpret the mc saying he’s “like a real son” if he actually was.
I mean there’s also a possibility that the uncle isn’t even blood related to mc, he could just be the husband of one of mcs parent’s siblings.
So what if they're related? So what?
I don't understand why it's the readers problem to help someone else justify their conscience about a story. I say let them read the story themselves to find out or leave the story alone if it bothers them. After all, there are literally millions of other stories they can read.
How will this story even be resolved?? They're cousins?? unless they find out they're not??