Jamieson, Fausset and Brown on James 3:10

10. The tongue, says ÆSOP, is at once the best and the worst of things. So in a fable, a man with the same breath blows hot and cold. "Life and death are in the power of the tongue" (compare Psa 62:4).

brethren--an appeal to their consciences by their brotherhood in Christ.

ought not so to be--a mild appeal, leaving it to themselves to understand that such conduct deserves the most severe reprobation.