My mom is hitting my little brother a lot and I'm really scared about it.?

I believe the reason my mom is doing this is because the stress of her job and the disappointment over my little brother's failure at getting high grades. Even though I know punishment is reasonable sometimes, I think my mom overdoes it because of her anger. Because of her stress, she is unreasonable and often creates misunderstandings between herself and my little brother. She always assumes my brother lies even though sometimes he is telling the truth. For example, my mom doesn't think my little brother is scared of her so she beats him more than she has to. Despite my little brother's screams of pain, she thinks he is acting and keeps on hitting him. I'm really scared my mom is going to far. She never was like that with me. She knew when to stop. My brother is also showing depression, suicidal thoughts, and anger at my mother. I don't know what to do and I'm too scared to talk to my mother. She also won't listen to what I say. She just assumes I'm babying him. I feel that my family is coming apart. My mom always complains how my dad never makes any money from being a mechanic because he is too generous and that she supports this house. She always complains how my brother is not high achieving like me. I feel like there is no one to depend on and nothing I can do. Please help me.