"Is Julia someone's wife?"

Translation: क्या जूलिया किसी की पत्नी है?

[PblerasPrimavera]

Why किसी instead of कोई?

[JosephYanc1]

किसी is oblique, meaning you use it with postpositions like का.

[ErwinRooij]

Thank you. And किसी is oblique of कोई?

[Gita-ji]

Yes.