Possible meanings are 1) that they approve of the thief's actions. AT: "you approve or him" or 2) that they join the thief in their actions. AT: "you join him" (See: [[rc://en/ta/man/translate/figs-idiom]])
God speaks of a person speaking evil things as if that person's mouth is a messenger whom the person sends to do evil things. AT: "You are always saying evil things" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-synecdoche]])
The word "tongue" represents the person who is speaking. AT: "you are always telling lies" (See: [[rc://en/ta/man/translate/figs-synecdoche]])
# You sit and speak against your brother; you slander your own mother's son
These two phrases have the same meaning but use different words. God accuses them of speaking falsely against members of their own family. (See: [[rc://en/ta/man/translate/figs-parallelism]])
# You sit and speak
To "sit and speak" against someone implies that this person deliberately thinks of bad things to say about someone. AT: "You always think of ways to speak" (See: [[rc://en/ta/man/translate/figs-idiom]] and [[rc://en/ta/man/translate/figs-explicit]])