Man I could've written your post!
Except he's not technically my brother, he's a best friend of ours (mine and my brother's) he's as much a part of our family as anyone related by blood, its been that way for over a decade.
What did we do? Well nothing we tried worked, so i strongly suggest you do nothing... He even got her pregnant and I nearly lost my damn mind.
I was so upset at the prospect of having the succubus of satan in our lives forever.
Luckily he eventually realized we were not just hating on her for no reason when he found out she was cheating on him.
And even then, he gave her a second and a third chance. When he finally realized she's really not gonna break things off with her boss, that's when he ended the relationship for real.
Watching him torture himself like that was brutal, we actually ended up giving him an ultimatum.
Said he can hope she magically sprouts morals and dignity, but he can do it alone, because we're through watching the same train wreck over and over and having to pick up the pieces repeatedly.
She still pulls his strings tho, via their son.
"If you don't do so and so, you'll never see him again, we're better off without you" blah blah blah, then she calls him three days later for more money or whatever.
His problem?
Zero self-esteem
Severely conflict avoidant
Crippled by his fear of change
Scared of being alone.
I mention this why?......
Because Rose has a point, something is up with your brother to be accepting her and all she comes with.
She's not holding a gun to his head.

I'm really sorry you're forced to watch this, IDK if I could do it again.
It was 3 years of hell.
I even started to resent him at one point because our trio no longer existed, no more movie nights and Saturday morning hikes. No more joining us for Christmas, birthdays, born days, baptisms, etc.
Why? Because she declined the invite, he wasn't "allowed" to attend without her. For over three years, none of us saw him for longer than 10 minutes at a time, and this guy practically lived in our house. (well he's "moved back in" now but you get what I'm trying to say)
I haven't completely gotten over his rejection of our family and his utter stupidity yet, but I'm working on it.