For what I understood he has a wife who is the empress but she is sick, he isn't taking concubines for some reason so the princess who is his only legitimate heir has to take it in his place
Now some personal theories:
was og princess encouraged/forced to "take noble man" for the stability of the empire with the promise of being empress one day? When did they start to force drugs on her? Could it be that it uses was for the sake of "being ready"? (Cuz this changes my perspective of her)
Second, the emperor doesn't want to or CAN'T take concubines? It's said somewhere he is infertile (or the queen is...but it can also be a lie to cover for him) IF he is infertile, it would make sense that the empress/nobles would block him from having concubines just for fun thus the princess would be a tool and a source of annoyance and treat for him
If he can't take concubines, is that the reason he is lying that the woman he fell in love is a lost sibling? Also so he could "leave his heritage" to someone of his choice (in that case, does he love that bitch for some beakground or she is conveniently "a option that I can chose" for him?)
Isn’t the king also supposed to be Elvia’s brother why is he so cruel to her?! Taking the side of some rando girl who’s trying to sleep with him, be his sister, and princess all at once?! ヽ(`Д´)ノ