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 "whether you always want to sin and so do sin ... or you want to obey and are able to obey" (See: [[rc://en/ta/man/translate/figs-personification]])