What if the author throws a curveball and Sumin is actually the one, who's gonna die.

I thought so too. Because the doctor in one of previous chapters said so. Sumin will die young, if she can't maintance her health. But, if sumin died, Seju will also die too (because Sumin is her anchor). It's like Romeo and Juliet. Tragic ending... I hope Sumin is okay, and hope she will happy with either Sungji or Seju, I don't mind. Hehehehe