Hera had the two brothers drop dead instantaneously as the best thing she could give them was for them to die at their moment of highest devotion.
The most often used quotation from this episode is (roughly translated) "call no man blessed until he is dead.
"[1] Herodotus, the original source for this story, does not state the name of the mother of Cleobis and Biton.
133 (= Stobaeus 4.52.43) "ἔτι δὲ Κλέοβις καὶ Βίτων, Κυδίππης τῆς μητρὸς αὐτῶν ..." Plutarch (first century CE) is the earliest source for her name that is now available to us.
Surely much intervening literature regarding Cydippe the priestess of Hera has been lost, since Plutarch was writing about 500 years after Herodotus first told the story.