The Bone Brethren item set appears to be bugged: While items cannot be equipped individually (as is proper), they lack the typical colored description text and cannot be equipped even when all three are in inventory.
The problem may be that there are two "sets" of Bone Brethren items. I saw two pieces recently that were coded correctly, but the armor piece was entirely different from the one I'd seen before.
Found the typo.
Future drops should work, feel free to contact a DM to get a replacement for the broken armor one.