I guess the question that would concern me if it were my brother is how honest brother has been with his fiancee. Like, any marriage arrangement should come with honesty, whatever the agreement in how they're going to live out their marriage.

After that's been established, whatever configuration they're going to live in, I'd back them up in it to the judgemental parts of the fam.

But, yeah, if the brother hasn't been honest (with himself, with his fiancee) then I'd advocate honesty. And if he's not going to be...then at that point, it's just supporting and working out where the lines are drawn regarding who keeps what secret and what is going to happen when (not if) things come out of the closet and start having to be dealt with.