Which fiduciary interests are those?

Please be more specific, since I can't figure out exactly what your objection is to a freeloading brother who isn't bothering anyone in the building, doing anything illegal (that we know about), or breaking House Rules (that we know of).

Bottom line to ME seems to be that your board is unhappy with the super's performance, and this is "icing on the cake." So instead of being honest about your dissatisfaction with his work, perhaps telling him face-to-face, you're using the fact that his brother is living there as an excuse.