Agrippa II, Brother of Vespatian

Yes, I'm now thinking that Agrippa II was a younger step-brother or even half-brother of Vespatian. The casual hosting of Vespatian by Agrippa might be consistent with that relationship. By any means, the two seem to be wining and dining as the war is prosecuted - it was "business as usual" for the royal family.

Interesting that Trajan is also mentioned in the passage. He too must have been a close relation and therefore essentially a "Julio-Claudian" in order to have gained the succession after Vespatian and his "sons". Neither Trajan nor his successor Hadrian had a true son to succeed them, which further points to their being maximally inbred members of the royal family.