What can I do to help my Mom in her emotionally abusive relationship?

I recently moved back in with my mom, her husband and his son about 4 months ago. In that 4 months I have witnessed my stepfather throw glass mugs, and objects towards my mom and myself. He has said to her to "go get drunk and crash the car into a tree." "Go overdose on painkillers and sleeping pills." He has called her the c u next tuesday, b****, and anything else you can think of. He use to hit her, but she tells me he hasn't in a very long time and I have not witnessed any physical fights since living here. My mom is stuck, they have been married for 6 years and has noone to turn to, she hasn't worked in about 3 years, and cannot drive.
All of the fighting takes place while my stepbrother is not in the house. He is 18years old.
I have confronted my stepdad on his behavior and he feels he has done nothing wrong, currently we are not talking but my mom is kissing his a**. He refuses to see a councelor.
How can I help my mom? I have noone myself to turn to, I have moved from place to place and do not have any friends. I am looking for work and in July I am leaving for basic training. But how can I leave my mom in a situation like this? If you have any ideas or advice please help.