Was Kuber really brother of Ravana?

There are two different stories

Sage Pulastya was son of Brahma and he married daughter of sage Trunabindu and blessed with a son Visravasu. Visravasu married daughter of Sage Bharadwaj and blessed with a son by name Vysravana (Kubera). Later Vysravana performed penance and Lord Brahma made him one of rulers of 8 directions and also gifted pushpaka vimana.

Other part of the story.

There were two demons called Hethi and Prahethi. Hethi had a son by name Vidyutkesa. He was blessed with a son Sukesa. Sukesa had three sons namely Malyavantha, Sumali and Mali. Viswakarma built Lanka and gifted to them. Thus the tribes of Rakshasas developed their population and they began harassing people. Lord Vishnu declared battle against them and Mali and so many of his associates were killed. Malyavanta and Sumali fled from Lanka and hid in Patala.

At this stage, Vysravana (Kubera) son of Visravasu occupied Lanka.

After some time Sumali came out along with his daughter Kaikasi. He observed the glory of Vysravana and advised her daughter to go to Visravasu the father of Vysravana and manage to marry him.

Thus she married Visravasu and blessed with three sons and one daughter namely Ravana, Kumbhakarna, Soorpanaka and Vibheeshana.

Later Ravana and his brothers performed great penance at Gokarna and Brahma blessed with different boons.

Thus Vysravana (Kubera) was brother of Ravan.

This story was narrated in Uttarakanda of Valmiki Ramayan.

Source – Valmiki Ramayan – Chapter Uttarakanda.