"Nor bridegroom's voice e'er heard shall be.
In silence ever shall thou lie,"
This means the woman never had - and never will have - a husband.
"The body where he sets his heel
Quail from your downward darting kiss."
I think we need a little more context on this one.
About "Sister Steel" - weapon (rifle):
"She glitters naked, cold and fair."
"Cold" doubles here for cold in temperature (being naked, and being metal, it would be cold to the touch), as well as impartial or emotionally distant. "Fair" is usually, in poetry, meant to describe a person's appearance - it can mean either pale, or beautiful. I'm not familiar with this poem, but I suspect "fair" is also doubling here as "just," making the weapon a sort of impartial judge.