NMom and GC’s abusive gf

My siblings are all in our 30s now. Unsurprisingly my GC brother developed into an asshole and as a result is only able to keep other assholes in his life. We met his gf “May” in college ~15 years ago. We hadn’t spoken to her since but about 2 years ago she made a reappearance in his life and they have been dating since.

From the start of the 2 years their relationship has been in turmoil. He is paranoid and jealous, and she is all sorts of crazy (mental/physically abusive, suicidal, manipulative, etc.). She has also exasperated his drug use from smoking we’re to doing coke regularly. He has lost the few friends he has and I myself have gone minimal contact during this time.

Over the past few months their toxic relationship has escalated for the worse. She was arrested and jailed for attacking him with a knife. My brother went out and laid nailed the next day. Then a few weeks after that I saw him again and he had scratches and bruises on his face. I express my concern to him and his response was to downplay it and say stuff like “yeah she gets like that.”

Even though my brother has been a huge asshole all my life I still concern for his safety. At this our parents and other sibling know what is going on. We have tried talking to my brother but he isn’t trying to hear it. Although the situation sucks he is an adult and can make his own decision. However, here is where my NMoms reaction comes into play.

The first time we talked about my brothers abuse she feigned concern and she’s some crocodile tears. Shortly after you’d see her tagged in pics with May having a blast. I called my mom and said by hanging out with her she’s accepting May into the family and condoning the abuse. She assures me they are not friends and the only reason she spent time with May is cause my mom was bored and she reached out.

A few weeks ago they had another domestic incident where she tried to choke my brother over something stupid. The cops were called but he didn’t want to press charges and decided to spend the night at our parents even though he owns the house they live in (she doesn’t work). I went over to talk to him but he wasn’t trying to listen and made light of the situation. I spoke with my mom and her response was baffling.

She told me if I was being a bad sibling for not supporting his relationship and accepting May into our lives. That I need to make May feel welcomed and maybe she would not hurt my brother as much. I couldn’t believe what I was hearing. I told her by accepting May into our lives we are encouraging the abuse that will eventually lead to more serious injuries. NMom started crying and telling my to stop blaming her for Mays actions.

Now NMom is telling people that I’m an asshole blaming her for my brothers situation. That I’m not supportive and driving a wedge between the family. Essentially making my concerns about herself. Another frustrating aspect of this is that I’ve been with my SO going on 8 years and she’s been nothing but sweet and nurturing. Of course NMom has talked shit the whole time bad mouthing her when given the opportunity. I take this situation as another example when being the GC is detrimental because they are not really cared about, but instead an accessory to the narcissist.