Wow.
There are a few differences in your story that makes me feel confident that you're not my BIL talking about me and my RAH. Phew!
Okay, my RAH is the youngest of 5 and in a lot of ways, being treated like a kid is what lead him to drink in the first place. When my husband first started going off the rails I called my BIL and my MIL & FIL to help me out with my husband because maybe they could get through to him, because I couldn't and was pregnant with our second child ie: exhausted and hormonal. Leaning on his family members only exacerbated the situation and progressed his alcoholism greatly. Further, its driven an irreparable wedge between his family and myself and our children (although, his mom is crazy manipulative and controlling, so we have some other stuff going on that you may not.)
You know what our current solution is? My husband just rented his own apartment. He literally moved in there yesterday after staying in a hotel for a month. He was really pushing back on me about doing it and was brought to tears that the lease was 6 months rather than month to month (he's afraid to live alone.) He's never lived alone and he's always had someone else making decisions for him. He's also attending AA daily (tomorrow is day 30 for him), seeing a therapist twice weekly and now living solo.
I think you're on the right track but it might be your SIL (ex-SIL?) that you need to convince of this. It sounds like your brother either loves her still or is co-dependent on her (my husband is codependent on me too) and her opinions/suggestions probably hold more weight with him.
Good luck. I hope your brother sorts himself out for the sake of his kids.