Kubera, the god of wealth is a (step) brother of Vibhishana. Vibhishana is always inclined towards righteous.
Consider the following words of Kubera
Kubera said— Vibhisana is still against to me. He does not abandon the family protest against me. It is a fact that the wicked seldom please if someone does any good for them. They remain as hard as the thunderbolt of Indra. It is seen that sanguine relations never please with virtues,assistance or friendship too.
[53-54, CHAPTER 63: Indra's rescue from the female sex, Narasimha Purana]
From the words of Kubera, it is clear that the relation between the Kubera and Vibhishana is not well. What is the reason for it? What unrighteous did Kubera do?