Sumali is the grandfather of Ravana

Ravana

' is the primary antagonist character of the Hindu legend, the Ramayana; who is the great king of Lanka. In the classic text, he is mainly depicted negatively, kidnapping Rama's wife Sita, to claim vengeance on Rama and his brother Lakshmana for having cut off the nose of his sister...

, the principal antagonist of the Ramayana

Ramayana

The Ramayana is an ancient Sanskrit epic. It is ascribed to the Hindu sage Valmiki and forms an important part of the Hindu canon , considered to be itihāsa. The Ramayana is one of the two great epics of India and Nepal, the other being the Mahabharata...

.

He, his elder brother Malyavan

Malyavan

Malyaban or Malyavan was the Chief Royal Adviser to the Emperor of Lanka, Ravana. Malyavan was a respected voice within the intellectual and royal circles in Lanka...

 and younger brother Mali were the sons of the daitya

Daitya

In Hinduism, the Daityas are a clan or race or Asura as are the Danavas. Daityas were the children of Diti and the sage Kashyapa. They were a race of giants who fought against the Devas because they were jealous of their Deva half-brothers...

 Sukesh.
Sumali along with his two brothers made a tapasya to Brahma

Brahma

Brahma is the Hindu god of creation and one of the Trimurti, the others being Vishnu and Shiva. According to the Brahma Purana, he is the father of Mānu, and from Mānu all human beings are descended. In the Ramayana and the...

 and asked that their brotherly love would be unbreakable and no one would be able to defeat them. Consequently they conquered every god and Daitya. With these powers they asked Vishvakarma to make a city for them.