There are several things going on in James 3:6. Two of the more interesting are the placement of γλῶσσα and whether καθίσταται is a middle or passive.
“And the tongue is a fire (ἡ γλῶσσα πῦρ)! The tongue is a world of iniquity (ὁ κόσμος τῆς ἀδικίας ἡ γλῶσσα) set among our members (καθίσταται ἐν τοῖς μέλεσιν ἡμῶν ); it defiles the whole body (ἡ σπιλοῦσα ὅλον τὸ σῶμα), sets on fire the course of our life (καὶ φλογίζουσα τὸν τροχὸν τῆς γενέσεως), and is set on fire by hell (καὶ φλογιζομένη ὑπὸ τῆς γεέννης).”
If the second γλῶσσα goes with the preceding, it means “The tongue is a (or “the”) world of iniquity, and this means that the subject of καθίσταται is drawn from its personal ending.
If γλῶσσα goes with the following, then κόσμος τῆς ἀδικίας is a noun phrase in apposition to γλῶσσα (“The tongue is a fire, a world of iniquity”) and the second γλῶσσα is the subject of καθίσταται. “And the tongue is a fire. The tongue, a world of unrighteousness, is placed among our members” (CSB).
I will phrase this verse in a video to lay out it’s structure.
But the more significant challenge is the voice of καθίσταται.