It's paranoia. The older brother is convinced Octavia hates him and is trying to ursurp his succession for the throne. The ongoing rumour is that Octavia is a likely runner-up to the throne because the king doesn't favour the younger brother. Also, some assassins who tried to kill older brother's boyfriend, Syl, claimed Octavia sent them (she didn't). He thinks she's trying to threaten him to give up his succession rights.