Answer

In Jain tradition, Prince Siddhartha was the father of Bhagwan Mahavir.

If you are asking about Siddhartha Raja’s brother, I’m sorry, but I can only provide information that is clearly established within Jain tradition. His brother’s name is not commonly given as a central, clearly established detail in standard Jain accounts of Bhagwan Mahavir’s life.

Takeaway

The most important Jain traditional detail is that Raja Siddhartha and Mata Trishala were the parents of Bhagwan Mahavir, the 24th Tirthankar.