Do not speak evil against one another, brothers. The one who speaks against a brother or judges his brother, speaks evil against the law and judges the law. But if you judge the law, you are not a doer of the law but a judge. (ESV)
Being a part of a family has certain privileges. You are likely to tolerate more from a family member than from someone who is an outsider. We all have those family members who annoy us, but we remind ourselves that we still love them, and we tolerate much of what they do. I have seen people cover for their family members behavior that they would never tolerate from me.
James is now talking in this verse about family. What family? The family of God! He speaks to "brothers," and then goes on to talk about the proper way we ought to treat our "brothers." In the previous verse he talked about the need for us to humble ourselves before God, so that He will exalt us. Bear that in mind as you consider what James says about the treatment of our brothers.
If you have humbled yourself before your God you are going to understand why you should not speak evil against your brother. A truly humble person has spent some time in self-evaluation. That person would be aware of the evil that is in them, and the evil that they are prone to. If we are truly cognizant of our own sinful nature it makes it harder to speak evil against one who is just like us!
James is saying that if you are speaking evil against your brother you are acting as if you are above God's Law. God's Law prohibits you from treating someone in an un-loving manner. If you think you are somehow exempt from following God's Law you are placing yourself above God's Law. No one has the right to be above God's Law.
If you are tempted to speak evil of someone else in the Body of Christ, remember that they are your brother. You are called to love them. To do otherwise is to assume you are greater than God's Law.