This question emphasizes how serious their previous sin was. This can be written as a statement. Alternate translation: "We had already sinned terribly at Peor!" (See: [[rc://en/ta/man/translate/figs-rquestion]])
This can be stated in positive form. Alternate translation: "we are still dealing with the guilt of that sin" (See: [[rc://en/ta/man/translate/figs-litotes]])