The first thing the title does is set our teeth on edge. Any time we get "death" or "dead" in a title, it's a heads up to cue the somber music. "This Hour and What Is Dead" immediately tells us where our speaker's mind is: focused on loss and death. So when we get to the brother and father, it helps us quickly draw the conclusion that these two are dead, and we're in the realm of memory and imagination.
What of "This Hour"? Based on the rest of the poem, we think it's safe to assume that our speaker is wide-awake in the middle of the night. So there's something about the wee small hours of the morning that cause this speaker to think about death. And not just any death, but the death of his loved ones.
Plus, the title also sets up a refrain in the poem. Bonus! The title, with a slight modification ("At this hour, what is dead is…") occurs three times in the poem. Three. By the time we get to even just the first occurrence in the poem, it rings a familiar bell. From there, the repetition of the title's refrain builds momentum, until that final occurrence, where it really punches us in the gut.
In the end, one thing is clear from this title: at this hour, the dead are gone. But their memories remain, and that's what haunts our poor, beleaguered speaker.