It is not certain that his followers are actually his relatives. The term used is literally brothers,
which can mean either siblings or companions. We cannot know for sure. But one way or the other, what has turned up in Shechem is another Abimelech, the only difference being that Abimelech himself turned up in Shechem alone and had to ensnare the oligarchs into providing the funds to hire his desperados, while Gaal shows up already in possession of his gang.
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26 And Gaal the son of Ebed moved into Shechem with his relatives, and the leaders of Shechem put confidence in him.