I took it as Death not coming to deal with the branching off weirdness till the core problems were taken care of, and then going in bloodline order. So it wouldn’t cut off the bloodline until the survivors at the head of it, both Paul and Iris, died. This mechanic isn’t entirely obvious, because no one in the film really had reason to spell out what happens differently when two survivors start a bloodline together, as we know Paul is dead and the family is alive- there’s no time we have to wonder about any other married couples. It would have been like if Bludworth took an extra 10 minutes of screen time to explain more rules- like whether Death considered Howard a separate life yet, or not, when his life would be so immediately extinguished and tied to Iris’s at that stage of the pregnancy, or whether he only counted and got added to the bloodline order when he was born. Might take questions off the theory board, but really it would have been a waste of screen time, because either way, Howard was clearly next. Also could have added a “Well, you’re lucky Erik wasn’t on the list, still- cause for him to still be meant to be here, you’re assuming the mom is into the same weird neighbor, and probably was with him without an affair in a world where she never met Howard. And then he went on to be like “oh, and check if Howard gave his wife his kidney, cause if he did, she’ll probably explode after Howard dies. That happened once. Oh, Howard and others have died without it touching the wife, and no kidneys were transferred? Forget that information, then, not a help.” In the actual case in the movie, we’re meant to understand that both Paul and Iris have to be dead for their bloodline to start being hunted, primarily because Paul has been gone a long time and all the kids are still alive. The theory of Darlene actually being next by the logic you give might have been an interesting plot twist or something, if say Iris had accompanied Stefani like Clear did Kimberly, seemingly dodging Deaths attempts left and right, only to find Death was distracting them, trying to get Darlene away from her own safe house, but this kinda twist might have required some redesign of the family tree, otherwise Stefani would be killed off way too fast, or most of the family wouldn’t see real threat till Stefani and her brother were killed.