I don't think there are general rules, but I do find it weird that anyone would take offence at not being mentioned in someone's obituary. The obit, after all, is there to describe the deceased and his/her importance in others' lives. The names of relatives are simply at the end, signature-like. I generally take the names at the bottom to mean that it is they who composed the obit. If his brother had not been involved, no reason why he should have been mentioned. He could have put his hand in his pocket and write a separate obituary, or come to your wife and ask to be included (it is not uncommon for multiple obituaries to appear for the same person, written by different relatives/friends etc).

As the uncle did neither, he has no right to feel aggrieved and does his own brother's memory a disservice by grumbling about it.