Paul uses a question to scold anyone who may think God's grace is a reason to keep sinning. You can translate this as a strong statement. Alternate translation: "You should know that you are slaves to the master you choose to obey!" (See: [[rc://en/ta/man/translate/figs-rquestion]])
Here, Paul speaks of "sin" and "obedience" as if they were masters that a slave would obey. Alternate translation: "whether you are like slaves to sin ... or like slaves to obedience" or "You are either a slave to sin ... or you are a slave to obedience" (See: [[rc://en/ta/man/translate/figs-personification]])