His relationship with Seju is unknown. All we know is that he's tied to her family somehow (cousin, step-brother, half-brother, fiance, etc.), and if you read the dialogue in the beginning, he seemed to be a bit over the top crazy with his love for Seju.
After I read chapter17, I'm so curious about what made Sumin think about Seju while she's doing things with red hair. Did that girl resemble Seju or something else? And why did she throw up after that?