You definitely have some merit in the way you feel.
Your wife, along with this "brother" is totally disrespecting and circumventing your authority,boundary, and space. They have, in effect dismissed you as the boys father.
If it were me, I'd show up after one of their weekend meetings and demand to meet with the whole body of elders and point blank them on what is going on and ask if they would tolerate such disrespect in their own home.
Play the headship card.
Barring that, I would withdraw my "ble$$ings" and start proceedings for kicking her ass to the curb for disrespecting me in such a way. If she and the brothers want to go there, I'd make damn sure they would be holding the bag for paying for it.