Yes, of course Christians sin at times, but we are not slaves to sin. In other words, sin doesn't control us as it does non-Christians. Paul wrote in Romans 7:17-20 that "But now it is no longer me doing it, but sin that lives in me. For I know that nothing good lives in me, that is, in my flesh. For I want to do the good, but I cannot do it. For I do not do the good I want, but I do the very evil I do not want! Now if I do what I do not want, it is no longer me doing it but sin that lives in me." That is describing the behavior of a non-Christian.
Christians live in our earthly bodies until we die and go to be with the Lord. We still sin on occasion, but not willfully. And when we do, then we repent. But our behavior is not determined by the sinful nature that we inherited from Adam. Romans 6:11-12, "So you too consider yourselves dead to sin, but alive to God in Christ Jesus. Therefore do not let sin reign in your mortal body so that you obey its desires". Clearly, Christians can control their sinful behavior (as free people), whereas non-Christians cannot (since they are slaves to sin).