What is the human relationship in the poem "Home Burial" by Robert Frost?

There are three relationships in Robert Frost's poem "Home Burial." The most evident relationship is that of the husband and wife. Another relationship is between the wife, Amy, and the couple's deceased child. The third relationship is between the husband and the deceased child. Of these, the primary "human" relationship is that of the husband and wife.

The relationship between Amy and her husband is rife with tension, as each is grieving the loss of their child differently. Amy stands at an upstairs window and watches the grave, unable to leave the house or talk about anything else. For Amy, time has stopped. Her husband, on the other hand, is grieving through action: he dug the grave and buried their child. He talks about the weather and its effect on their fence. As they talk, he stands at the bottom of a staircase, staring up at Amy.

Amy cannot understand how her husband can move, talk, or even think about anything other than their deceased child. She says,

You could sit there with the stains on your shoes

Of the fresh earth from your own baby’s grave

And talk about your everyday concerns.

Her husband, on the other hand, cannot understand how it is that Amy won't communicate her thoughts and needs to him.

‘Don’t—don’t go.

Don’t carry it to someone else this time.

Tell me about it if it’s something human.

Let me into your grief. I’m not so much

Unlike other folks as your standing there

Apart would make me out. Give me my chance.

At the time the poem takes place, the two, presumably once very much in love, are each other's worst enemy. Amy seeks understanding and wants to leave the house. She feels trapped with someone she feels can never reach the depth of her grief. The man, undoubtedly angry about the death of his child and the powerlessness that comes from not being able to console his wife, turns to intimidation as his only remaining tactic to get Amy to confide in him. 

"I’ll follow and bring you back by force.  I will!—"

When the child died, the relationship each parent had with that child was severed in the physical realm. The implied outcome of the final dialogue between Amy and her husband is that the relationship between the two of them will follow suit and will also be severed as the irreparable harm of the death of their child tears their marriage apart.