in mythology kubera the lord of when shared what relationship with ravana
He was his demon half-brother.
Kubera

Originally described as the chief of evil spirits in Vedic-era texts, Kubera acquired the status of a "Deva" (god) only in the "Purana"s and the Hindu epics. The scriptures describe that Kubera once ruled Lanka, but was overthrown by his demon half-brother Ravana, later settling in the city of Alaka in the Himalayas. Descriptions of the "glory" and "splendours" of Kubera's city are found in many scriptures.
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Kubera [SEP] Introduction
The scriptures describe that Kubera once ruled Lanka, but was overthrown by his demon half-brother Ravana, later settling in the city of Alaka in the Himalayas. Descriptions of the "glory" and "splendours" of Kubera's city are found in many scriptures. Kubera has also been assimilated into the Buddhist and Jain pantheons. In Buddhism, he is known as Vaisravana, the patronymic used of the Hindu Kubera and is also equated with Pañcika, while in Jainism, he is known as Sarvanubhuti.