Chickpea, I had to laugh....not at you or your current situation. I'm laughing at my own history. My sons grew up without sisters, it was a male dominant house hold. I grew up without sisters and watched my brothers make the same wrong choices.
Yes, I have kept my mouth shut, but continued to instruct them both about behavior that suggests problems. Slowly but surely they learned, as my words of wisdom came to fruition . There was one instance where I had no tolerance. I was on my son's computer and knew the girl he was seeing at 16 was a user. She started instant messaging thinking it was my son. I told her knock it off it was his mom on line. She refused to believe it and continued with the most horrific, ugly, nasty talk.
I promptly ripped her a new one....that was the end of the relationship....! (I paid for that for awhile, but got a "thanks mom" later)
My oldest son, with his last two relationships, found wonderful, darling girls. The latest is the sweetest,.... and I am sorry to say, I am losing him to overseas to be with her, but he could not have chosen a better girl and I wish them the best. He met her on a train in Germany. She comes from a small town in Bavaria, strong family, capable, working on her Masters in Vienna.
My younger son is still learning. Poor fella, he can't stand drama, and tantrums, and spoiled brats....which leaves little to choose from.
All you can do is give hints that this or that is going to wind up being a problem without referring to the present girlfriend.....wait, sit back and squirm.
Chickpea, just wait it out....it will turn out ok....you sound like you are a great mom, and yes, do not facilitate, or get involved unless necessary.
r.