Agrippa screws up the invasion of Sicily, gets killed (not so farfetched, it was his first naval engagement. OTL is more ASB than alternative history?). Sextus Pompey captures Octavian and his fleet, makes deal with Lepidus. Octavian gets executed as an enemy of Rome/the Senate, Lepidus and Pompey make some pro-senate statements to get support and now provide Rome with much needed grain from Sicily, becoming much more popular.
Both of them are supreme after Antony's farcical Parthian campaign, they find some pretext or other to goad Antony to war, have their own version of Actium, bye bye Antony. Cleopatra and Caesarion might be spared here, as long as the Ptolemies ackowledge Rome's superiority.
Lepidus as the more senior man is the supreme ruler of Rome, Sextus Pompey is his right hand.
