Only three Jupeater restaurants run an actual dedicated brunch service, as opposed to just opening early, and each does it differently enough that it's worth knowing which before you commit a Sunday morning to it.
Guanabanas runs its brunch dockside at the Jupiter Inlet from 10 AM to 2 PM on Sundays, with the tiki-bar atmosphere intact even in the morning. Ruth's Chris pairs a more traditional Sunday brunch service with its usual dinner hours later the same day. Sea Racha's brunch is a weekend add-on to its regular sushi-and-seafood menu, so expect a lighter, more improvised spread than the other two.
Because this list is short, check each restaurant's current hours directly before you go - brunch service is the most likely thing on a menu to shift seasonally, and none of these three run it every day of the week.