Nigel was trying hard not to show it in his face that he had already broken a rule of the clan. Adeline had already told him some gruesome details of what the clan did to keep their secret safe. And as Adeline was an outsider who knew the secret of the clan, he didn't want to give himself away and put his sister in danger.
Though Nigel was not showing any nervousness on his face, Rhea's sharp ears were able to pick up the sudden escalation in Nigel's heartbeat. But she assumed that it was because he was smitten by her, it happened most of the time when she would talk to men. So she carried on explaining the other rules to the Prince without thinking much of his escalating heartbeat.
