What can I do if my brother is 3 years older than me and is really abusive to me and my mum?

I dont know what do to :
My brother is 17 nearly 18 and he has threatened to hit me and has hit my mum, she now has big bruises on her arms and when people ask wat happened she says she hit it on a coffee table. She wont tell my dad because shes scared he will hit him or kick him out. I dont feel safe around him and want him out, i have been thinking if he doesnt move out then i will and im only 14, 15 a few days after he turns 18. So im too young to legally move out. I havent told any of my friend about whats happening except for my boyfriend but even with him i have only told him half of the story, I’m scrared if he knows what really happens in my life he will dump me :/
What can I do?