Narrative comes from the Aeneid book VI.
Note that there are two versions of this subject in Budapest, their inventory #s 551 & 553. They are frequently confused in the literature, including in Ertz 1979. 551 (the present painting) is the one where Sibyl is in the lead and 553 has Aeneas in the lead.
Previously it was thought that the figures were by Rottenhammer. that was Gerson's opinion; Freedberg too is not totally convinced by the attribution of figures to Brueghel. I believe them to be by him.