I’m not aware of anything in the Codex Juris Canonici that stipulates who is or is not entitled to use the titles of “brother” and “sister” — but theologically, these titles actually are proper to every baptized Christian. With God as our adoptive father, we all are siblings in our Lord, which is why religious orders, in seeking to live the gospel more fully, started using them.Naturally, a pastor may often address us as "my brothers and sisters" in a homily, and this would not be inaccurate or in any way reprehensible. Nor, as far as I'm aware, is there anything in the canons that prevents me from wearing a Franciscan habit if I choose, especially in the back yard. Nor, as far as I can see, especially if I can come to mass in shorts, flip-flops, and a torn t-shirt, is there anything that prevents me, canonically, from coming to mass in a Franciscan habit.
However, in the case I covered here of the ACA priest who styled himself OSA though he had never been an Augustinian, the regional superior of the Augustinians whom I consulted made it clear the guy was not entitled to say he’s OSA. And if I were indeed to go to mass dressed in a Franciscan habit and call myself “Brother John”, I believe Fr Bob would, er, take me aside, notwithstanding there’s no dress code and we’re all brothers and sisters.
Some things are misleading and could represent a danger to the flock even if the canons have no opinion.
