Wow.
Is her brother on medication for his autism? If he's not, he really needs to be. If he is, he needs it adjusting.
Autism affects the way you view the World. Instead of knowing automatically what is "right" and "wrong"... everything just seems to blend together. You can't empathise (which means you can understand others emotions) - and so this will account for him being nasty and violent.
I'm sure her mother will understand what is going on with her, so can you suggest that they contact the NHS to maybe see about getting a carer for him a few hours a week? This will allow them to have a few hours having a meal together or something, some time alone.
It's a horrible situation for you to be in, because now you're worrying about her. And it's a horrible situation for her to be in, because now she's worrying about him.
The only suggestion is that you stress to her that you're there for her if she wants a break from things. Suggest you go to the cinema a few times a week or something, to give her some time off.
Other than that, the only thing I can suggest is that you ask her to ask her mother to maybe see about adjusting his medication - because such symptoms of autism that cause violence should be quashed by medication. As well as this, maybe she can look into having a carer or a home visit therapist.
