Though the Edomites were closely related to the Israelites, it seems that when the Israelites needed their help the most, the Edomites did more harm than good.
The Edomites were the descendants of Esau, who was the son of Isaac and the twin brother of Jacob (Genesis 32:3). The Edomites settled in the mountainous region to the southeast of Israel (which was called Canaan at the time), driving out the Horites who were there (Genesis 36:9).
The shaky relationship between Israel and Edom becomes evident very early, when the Israelites were seeking permission from the Edomites to pass through their territory on their way to the Promised Land of Canaan. The Edomites denied them permission to pass through and threatened to attack the Israelites if they tried to do so (Numbers 20).
Years later, Saul and David fought against the Edomites, and David eventually brought them under the rule of Israel (1 Samuel 14:47; 2 Samuel 8:11–14). After Israel split into two kingdoms, the Edomites remained under the rule of Judah for a time and even helped King Jehoshaphat of Judah fight against Moab (2 Kings 3). But later the Edomites regained their independence during the reign of King Jehoram of Judah (2 Kings 8:20–22).
When the Babylonians attacked Judah in 586 BC, the Edomites displayed disloyalty to their relatives again. Instead of helping them, it seems that they capitalized on Judah’s weakened state and made raids on them as well (Obadiah 1:1–21). By the time the Jews returned from exile in Babylon, many Edomites had moved into southern Judah, and it became known as Idumea.
At the time when Jesus was born, Herod the Great was king of Judea, but he was not even fully an Israelite. He was an Idumean.