We're going to go ahead and state the obvious here: with a word like "Dead" in the title, this poem is probably more than a little about death. In fact, "This Hour and What Is Dead" is downright obsessed with death. Throughout the poem, our speaker's thinking about his dead brother and father, and about the enduring love and connection between him and both of them, which haunts him to this day. Then, to top it off, when God appears, he brings even more death. This is not a happy poem folks, so consider yourselves warned.
In "This Hour and What Is Dead," the living and the dead do not occupy completely separate worlds, which causes the speaker all sorts of distress.
In this poem, to die means to be forgotten. Since the speaker can't forget his brother and father, they're not actually dead.