Some early translations tell explicitly what Cain said to his brother. You may want to do the same if it is present in your national version. AT: "Cain said to Abel his brother, 'Let us go into the fields.'" (See: [[rc://en/ta/man/translate/translate-textvariants]] and [[rc://en/ta/man/translate/figs-explicit]])
God knew that Cain had killed Abel, but he asked Cain this question so that Cain would have to answer. (See: [[rc://en/ta/man/translate/figs-rquestion]])
Cain used this rhetorical question so that he would not have to tell the truth. This can be translated as a statement. AT: "I am not my brother's keeper!" or "You know that taking care of my brother is not my job!" (See: [[rc://en/ta/man/translate/figs-rquestion]])