In Faulkner's As I Lay Dying, Cora indicates that Darl has a different relationship to his mother Addie than his brothers and sisters do that she, for some reason, can sense:

It was like he would never see her again, that Anse Bundren was driving from his mother's death bed, never to see her in this world again.  I always said Darl was different from those others.  I always said he was the only one of them that had his mother's nature, had any natural affection.

From this passage, you can see that Darl, at least according to his sister Cora, has a different relationship to their mother, "different from those others."  Cora highlights this difference, but the exact nature of this difference only comes to the fore in his mother's only narrated chapter.  Addie--probably from beyond the grave--reflects on her marriage after she and Anse gave birth to Cash:

Anse or love: it didn't matter.  My aloneness had been violated and then made whole again by the violation: time, Anse, love, what you will, outside the circle.

In this passage, you can see that after having Cash, Addie was raped, or "had been violated," putting her outside the "circle" of the traditional family.  Addie continues:

Then I found that I had Darl.  At first I would not believe it.  Then I believed that I would kill Anse.  It was as though he had tricked me, hidden within a word like within a paper screen and struck me through it.

From the language above, we see that Addie feels as though Darl represents a kind of trick on Anse's part, signaling his exceptional place in the Bundren family.  Because Darl is the result of either spousal rape or infidelity, he is not on the same plane as his siblings.  

Darl's reaction to his mother's death makes him not as "queer" as most think he is because:

He come to the door and stood there, looking at his dying mother. He just looked at her, and I felt the bounteous love of the Lord again and His mercy. I saw that with Jewel she had just been pretending, but that it was between her and Darl that the understanding and the true love was. He just looked at her, not even coming in where she could see him and get upset, knowing that Anse was driving him away and he would never see her again. He said nothing, just looking at her.

The fact that, as the narrator recounts twice, Darl only stares at his mother indicates, for Cora, an intimate connection that goes beyond the relationship she has for her other children.  Furthermore, Darl's "just look[ing]" indicates that his grief upon his mother's death transcends words and actions--something that his siblings are unable to do.