I can't exactly say it's his lover Syl. On one hand you would think so because it's shown how much the crown prince cared for his sister so maybe Syl had some kind of twisted jealousy at best? But then that's the only motive as Octavia being out of the picture would only hurt him instead. She actively helps him not only as the.... (Ugh) "Babymaker", but she also helps him politically as the future king consort since that's her duty as a member of the royal family. She doesn't let herself be walked over but she still doesn't try to sabotage either of them when her brother treats her so poorly.
Yeah, that's why i don't really think its Syl either. Like, he wouldn't want the crown prince to take in a consort or mistress, so Octavia has to stay.
Sooo, the king told the escort knight to protect the mc at all cost cause she is really important for keeping the bloodline, but the crown prince is trying to frame her ( and maybe even kill her) cause she " Is hurting" His lover. Okay, boy. I know he is under some spell and stuff sooooo, who the fuck gains more from removing Octavia from the picture? His lover, the opposite political grup, the sky father?