EX is being abused by live-in BF. What to do?

Divorced 9 months. I have primary custody. My EX moved from Texas to Minnesota to find greener pastures. My EX came down during Spring Break and informed me that her live-in BF (I think he's an EX now?) had physically hit her across her head back on New Years Eve. Our kids and his kids were downstairs and did not see this. My EX turns around the next day and takes her and the kids to a hotel for the last 4 days during their visitation (it's true). She was scared to tell me at the time because of what I'd do. And this is one reason why the kids did not visit her during Spring Break. He's also verbally abusive to her. That's all she shared. Don't know how many times he has hit her. The BF has been out of a job since October and has a history of alcohol abuse. Not that it excuses his behavior. The BF also has a short fuse can explodes very easily. So, my EX had moved in with this BF when I decided to file for "D". They have been leasing a home since last May. So she's finally getting her own place (in the same neighborhood) in a few weeks. She stated the BF wants to move in with her. She said she won't allow it, but also commented that she worries for his 2 girls and that is one reason why she has stayed around for so long. Plus, she commented that moving in together was a good idea in her eyes and she just doesn't know where it went wrong. Why is she still living with this guy?!? I would have been out of there the next day! But she did admit she doesn't want this guy around our young kids. Don't worry. If this guy is living with her, my kids aren't visiting and she can try and take me to court on it. She also shared this with her dad back in February apparently. So here's my little problem. I am very close/ good relationship with her brother. He doesn't know about this. I'm very torn on this because on one hand, it shouldn't be my business to tell him. However, the fact that I do know puts me on a spot that if/when he finds out he'll be pissed that I did not tell him. What do I do? SHould I tell him? Or should I not? And if I don't and he finds out, how should I tell him why I decided not to say anything?

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