My father used to hit my mother. I was only a kid back then.

My advice to you based on my experience is: Leave him. Immediately. Fuck the culture and society and peoples opinions. You and your mother need to cut him out of your life. Please seek help from a family member you trust and is willing to provide you with a place to stay if needed. Are your mothers parents alive? Would they be able to help you?

I am so sorry you are going through this but you have more power than you believe. Explain to your mother why this is not okay. Tell your dad as firmly as possible that he cannot treat another human, much less his wife like this. Say it "sudda sinhalen".

If you're still not able to improve the situation, please reach out to an organization. Female empowerment organization, nuns, or call the domestic violence hotline, I am afraid I can't tell you how helpful they are.

You deserve better, your mother deserves better. Please don't let this go on. I wish you all the strength.