My mum is in a mentally abusive relationship with her husband, my stepdad. He calls us pigs and has really high expectations. Problem is they really love each other and I fear they might literally kill themselves if they don't make it through together. I figure the solution is for her to rent an apartment away from him for a while, with me and my 2 younger brothers, but shes fcking stubborn.

Its getting very serious, last month he texted her a picture of him holding a gun to his head and the words "Goodbye", just to see how she would react.

Please don't urge me to contact authorities, my entire family is already aware of the situation, I just want to know how I can do whats best for her.

Thanks.

Unfortunately their is not much you can do. the heart wants what the heart wants even if it hurts them. you could try to reason with your mom or something but that probably will not work you can talk to the police but if your mom protects him then it won't go anywhere. That being said after you take all other measures be ready to protect you mom at all cost make a plan hidden weapon catches know what you will do with it if the moment of truth comes but pray that you will never have to use it.

I agree with Edaneres. My father to is the same. I told my mother over and over to leave him. She never listened. Until she realized that she never needed my father. She only wanted him. But it was to late, to fix anything.
If it starts to get to a point where someone is physically hurting themselves or other, that when the officials step in. But just ask her what she needs from him. Like what does her give. And when I say that I mean emotionally.