My mom is leaving her husband and son

So, long story short, my mom is leaving her abusive husband and 15 year old son. They are actually all very verbally, emotionally and sometimes physically abusive to each other. Her husband is not my father and her son is my half-brother, who I was close with. My mom hasn't worked in 15 years, but I am hoping she can get a job and take back control of her life. I am sad to be loosing a relationship with my brother it seems, but I do not want her husband around my newborn, he is violent. I am mourning the family that we had and the person my brother could have been if he hadn't grown up in that abusive environment. I don't think it is right for someone to abandon their family/child, but my mom has tried every therapy, every resource available to her to help my brother and he continues to get worse (stealing, watching porn, being violent, never doing school work). I actually think my mom is strong for leaving after the deep depression she has been in.