That quote is often misused. Many people believe that it means we shouldn't point out another person's sin at all. However, if you read further Jesus says that you must first get your life in order so that you can better help your brother/sister that has stumbled. If you are constantly consuming to much alcohol and becoming drunk then how can you tell someone else that they shouldn't drink. Or if you are engaging in extra-marital affairs, how can you tell another that they should stop having an affair of their own? We are warned not to do that because the measure that we choose to use in our judgment of others will be used to judge our own actions. We must cease our own sins and follow Jesus' example before we can start pointing out the sins of others. Now, that is hard for many because breaking sins hold can be hard to do. However, who would have the better testimony for leading another to Christ:
1. A person that grew up and has never drank alcohol
2. A person that had struggled with alcoholism and addiction their whole life
Just because a person has sinned doesn't mean that they can't point out another's sins. If the only people that pointed out another's sins of alcoholism were people that had never had a drink then where would the power of the testimony come from. A person that has defeated a sin with the help of Jesus is a much stronger testament of God's power than someone that never committed that sin. God will use our sins to demonstrate His power over it when we allow him into our heart.
Matthew 7:1-5
7 Do not judge, or you too will be judged. 2 For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you.
3 Why do you look at the speck of sawdust in your brothers eye and pay no attention to the plank in your own eye? 4 How can you say to your brother, Let me take the speck out of your eye, when all the time there is a plank in your own eye? 5 You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brothers eye."