Everyone in My Family Has Killed Someone is similar to other humorous fiction I've read in that it is wry and clever but not what you'd call a knee-slapper. But I could see what contributed to the humor, which makes it a first with the humorous fiction I've, shall we say, studied.
Voice, tone, understatement, and structure all contribute to the humor of this book. You could say the narrator's inner life contributes to the humor. You could say there's some incongruity here, too, because how could someone like Ernest (the narrator) exist in that family? The structure, as I've already said, supports/contributes to the humor, and maybe you could argue there's something incongruitous going on with the structure. Ernest tells us he's a reliable narrator, that he will tell the truth. And he does, but in his time and not at the times the readers' expect him to. So we assume we know things, find out later we didn't, and are amused and amazed.
I did figure out some of the ending material with this book, but in a satisfying way. Which I will not get into, since I don't want to ruin anyone's reading pleasure.
This is a book that both people who like mysteries and people who don't care one way or the other should be able to enjoy.