Is it allowed to protect myself?

[Nawshin Ahmed]

I belong to a middle class Muslim family. My parents are separated right now. I live with mh mother with my younger sister. My mother has an affair to another man. When ever she has a fight with her boyfriend or my father says something against her wish, she gets furious. She starts to beat me and my younger sister. She uses wooden or iron sticks to hit us. She is cheating on my father. She goes to several places with her boyfriend secretly. They are in a deep relationship. My mother hate us now.
I want to protect myself and my sister from her. What should I do? Is it allowing to push her away or hold the stick for self defence?
Please help. We badly need help!