Raj is married to Priya, and they have one son named Aarav. Priya has a sister named Kavya who is married to Vijay. Kavya and Vijay have two children, a son named Rohan and a daughter named Diya. How is Diya related to Raj, and how is Aarav related to Vijay?

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Diya is the daughter of Kavya, who is Priya's sister. This makes Diya Priya's niece. Since Priya is Raj's wife, her niece Diya would also be Raj's niece by marriage. As for Aarav, since he is Raj and Priya's son, and Kavya is Priya's sister, making her Aarav's aunt, Vijay (being Kavya's husband) would be Aarav's uncle by marriage. Thus, Aarav is Vijay's nephew, and Diya is Raj's niece by marriage.