Bet he slept with her sis thinking it was her... LOL
truly tragic. but you did what you did bro
UNLESS, the sis is lying. Maybe she had an affair with some other dude and so she's lying to save herself. And MAYBE, the prince isn't remembering anything coz he got drunk one night and blacked out with the sis. nothing happened, but he doesn't remember that, thus the misunderstanding. just like a thousand other manga/manhua(specially manhua)/manhwa
Even tho this would be super cliche, i really hope this is it. Coz otherwise it'd just be sad and tragic.
Nuff speculating. Time for appreciating
Many thanks for translating.