Ram and Sham are real brothers. Sita and Gita are real sisters. Ram's son is Gita's real brother.
How is Sham related to Sita?

1. Ram and Sham are real brothers. It means they share the same parents. 

2. Sita and Gita are real sisters. So they also share the same parents.

3. Ram's son is Gita's real brother. That means Gita and Ram's son is a sibling. So, Ram is the father of both Gita and her brother. 

Therefore, Gita (and Sita) is Ram's daughter. Since Sita and Gita are sisters, and Gita is Ram’s daughter, Sita is also Ram’s daughter.

 So, Sita is Ram’s daughter, and Sham is Ram’s brother, so Sham is Sita’s uncle.