Seems like the stuck inability to communicate how one feels or to say anything of use at a hard time, in the poem covered by an almost ritualized Bible reading, because there are no other words than the scripture

That makes sense, and in the poems of Clifton's I've been reading she does use scripture as a vehicle of communication throughout.

I think I'd have to change my reading of the poem a bit to accommodate that understanding of the reference to Job -- right now I read the speaker's regard of the sister as quite tender, ironic but not cynical, recognizing a gesture of love in unlikely garb.

I wonder if it could be a kind of defiance on the part of the sister, or the poem, to say "none of us deserve this suffering."