He, to whom the gods ordain defeat, hath his senses taken away, and it is for this that he stoopeth to ignoble deeds. When the intellect becometh dim and destruction is nigh, wrong, looking like right, firmly sticketh to the heart. Thou dost not clearly see it, O bull of the Bharata race, that clouded intellect hath now possessed thy sons in consequence of their hostility to the Pandavas. Endued with every auspicious mark and deserving to rule the three worlds, Yudhishthira is obedient to thy commands.

In plain words: When someone is heading for ruin, his judgment goes, and wrong starts to look right to him. Vidura tells Dhritarashtra that his sons' hatred of the Pandavas (their cousins) has clouded their minds, while the worthy Yudhishthira, eldest Pandava, still obeys him.