Paul now specifically states about what sin of theirs he has heard, and how the Corinthian believers are proud of their acceptance of that man and his sin.
"Sleeping with" here is a euphemism for "commiting adultery with" AT: one of you is commiting adultery with" (See: [[rc://en/ta/man/translate/figs-euphemism]])
This rhetorical question is used to scold the Corinthians. AT: "You should mourn over this instead!" (See: [[rc://en/ta/man/translate/figs-rquestion]])