The Bible tells us of three sons that Adam and Eve had. Cain, Abel, and Seth were their names. Cain killed Abel but did not flee to his uncle. Seth was born sometime after Abel had been murdered.
Cain did not have an uncle because Adam and Eve did not have any brothers. Adam and Eve ultimately are the parents of every human. They could not have had brothers because any humans that arrived on the scene after Adam and Eve would have been their children, grandchildren, great grandchildren, etc.
The Bible tells us in Genesis 5:4 that Adam lived 800 years after Seth was born, and he begat sons and daughters. Adam and Eve could have produced many children in that 800-year span. And the children that Adam and Eve had would eventually produce children, and they would produce children, etc., and that’s how the human race got started.
In the beginning, the human race was allowed to multiply through close family sexual unions. Parents and children, brothers and sisters, etc., were allowed to have children together in order to expand the human race. In Moses’ time, thousands of years after Adam and Eve, God put an end to this practice (Leviticus 18).