My mom and stepfather's fighting?

Alright so my mom and step dad fight all the time. I live with my husband out of my mom's house, but my 2 sisters and 2 brothers still live there, and i am worried about how this is affecting them. My brothers are 18, and 14 and my sisters are 13 and 4. The 4 year old is technically my half sister, her dad is my step dad. But anyway, not long ago, my step dad pushed my mom and ended up really screwing up her ankle, she still has trouble driving and i think it was about 3 months ago. I don't think the fight was one sided, im sure my mom landed a few hits too, which in my mind only makes it worse. My 4 year old sister witnessed the whole thing, and my step dad spent the night in jail. Then just a couple weeks ago, my step dad ended up in jail again, i wont blatantly state the reason, but it was a kind of 'sting' operation, and didn't involve drugs, but in my opinion, I would NEVER have let him back into my house, or life EVER again. But my mom did. I flat out told her she was stupid for letting him come back, but then tonight i was over there with my 18 month old daughter, and they started fighting. I was outside with the baby and my sister to keep them away from the yelling, but i believe my step dad ended up throwing a coffee mug, and my mom would have been bashing him with a frying pan had my brother not taken it from her. I think this is a dangerous situation for my siblings, especially my youngest sister, however I'm scared of reporting anything, because i don't want them to stick her in foster care, and i don't know what else they would do. I refuse to send all my siblings to foster care because these two cant live together. It is my mom's house, she paid for it for 15 years alone before she met my step dad 5 years ago and he started helping. And he doesn't even work, so he really barely helps. I want him gone, and i don't know what to do. I think my mom would be furious if i took any measurement to get him out of all of our lives. It's horrible to hear a 4 year old say 'stupid dad, i hate him so much.' I don't know what to do anymore but im tired of watching my family suffer.