Hermie and Hestie never asked to be demigods.
Unlike others of their kind, they aren't interested in going on quests and being the center of a world-changing prophecy. But when Zeus learns of Apollo's vision--of the twins finding Prometheus, Zeus's number one enemy--the hunt for Prometheus begins.
Zeus and his allies want to bind Prometheus and to kill the twins before their threat of a revolution becomes a possibility. If Hermie and Hestie don't find the Titan first, they'll have no chance of fulfilling their destiny or of surviving Zeus's threat against them.