If Arun is the son of Raji's father, there must be brother-sister relationship between Arun and Raji.

**Arun's Relationship with Raji**

The given statement by Arun, "Though I am the son of your father, you are not my brother," suggests that Arun and Raji do not share a sibling relationship. To determine Arun's exact relationship with Raji, let us analyze the possible scenarios.

**Possible Scenarios:**

1. Arun and Raji have the same father, but different mothers:
- In this case, Arun would be Raji's half-brother. They would share a biological father, but have different mothers.

2. Arun and Raji have the same mother, but different fathers:
- In this case, Arun would not be Raji's brother, as they do not share the same father.

3. Arun and Raji have different fathers and mothers:
- In this case, Arun and Raji would not be siblings, as they do not share the same parents.

**Conclusion:**

Based on the given statement, it can be concluded that Arun and Raji are not siblings. However, their exact relationship cannot be determined without additional information about their parents and their respective family backgrounds.