Alexander of Epirus
Alexander of Epirus was the brother of Olympias. Like his Macedonian namesake he was said to have been descended from the hero Achilles (eg. Pausanias 1.9.8). He married his niece (Philip’s and Olympias’ daughter), Cleopatra, in 336 BC (Diod. 16.91.4; Just. 9.7.7)—it was during the celebrations of this wedding that Philip was assassinated. Philip had probably suggested the marriage in order to create a more direct tie with his brother-in-law—at least one ancient author suggests that Olympias was attempting to persuade Alexander to make war on her estranged husband (Just. 9.7.7).
While Alexander the Great invaded Asia, Alexander of Epirus went West, to Italy, in response to a call for help from the citizens of Tarentum. He died near Pandosia in Italy, in 331BC, while engaged in a war against the Bruttians and Lucanians (Just. 12.1-2; Livy 9.17.17; see also Frontinus 2.5.10, who describes Alexander’s skill at ambuscades).
Justin suggests that Alexander the Great was pleased that his brother-in-law had been killed (Just. 12.1; but see 12.3, which contradicts). It certainly seems as if there was some rivalry between the two Alexanders: Cleitus the Black is said to have taunted Alexander the Great with a quote from the Epirote king—that he had faced men in battle, while the Macedonian had only faced women (Curt. 8.1.37). That there was rivalry between the brothers-in-law might also be suggested by the fact that, when Harpalus first absconded from Alexander the Great’s camp (in 333 BC), his companion Tauriscus fled directly to Alexander of Epirus in Italy (Arr. 3.6.7).