I understand you sincerely want to help your brother. I truly do.
In that regard, you are like my father. He was a Fixer. His job in the company that employed him was, essentially, that of Fixer. My dad had a real talent for talking to the various employees and getting to the bottom of a problem, then working it out. In the working world, that was a great talent and it took him to the top of the company.
Unfortunately, he tried to apply his Fixer talents to his family, with very mixed results. It's one thing for the company president to say "OK, from now on it's this." It's a totally different thing for a father to tell his adult children "You should do that."
I had some real issues with my father, as much as I loved him, because he would try to fix my problems. Then he would be mystified as to why I did not do what he suggested. Why not? Because most of the time I only told him part of the issue. The rest was not his business. Or because I just did not want to do his line of thinking.
In retrospect, my father would have done better to stay neutral in certain issues. This is what I am advising you to do and this is what I try to do with my own children. I am loving and supportive, but I do not try to step in and solve their problems for them. If one of my sons was dating a woman that I disliked, I would do my best to try to find some kind of common ground. The last thing I would want to do is see some hypothetical future daughter-in-law coming to a message board such as this with something like "My FMIL hates me." I've seen a lot of that on this message board and in real life. I don't want that to happen to me.
I know you want to fix what you perceive to be a problem that your brother has, namely, an idiot girlfriend. I have a nephew whose past two girlfriends have been real losers. (Look up "dreck" in the dictionary and you'll see a picture of these two girls.) My brother, SIL and I all tried to be neutral towards these ladies, and I use the term "ladies" loosely. In both cases, Nephew has voluntarily broken up with them.