I understand what you are going through and can completely understand how you must be feeling now. You're probably thinking "Wtf? Why is my mum doing this?" and that is perfectly one hundred percent alright to think like that. You have to understand that you're not going through this alone, lots of people are experiencing the same thing you are experiencing now and I admire yourself coming to talk and seek help about it, it's very brave of yourself.
First of all you need to talk to your mum, I know you said she said she doesn't want to talk about it - but that's not good enough. Take her out somewhere, go grab a coffee or some lunch and just gently lean into the conversation, she'll be so relaxed she might just open up - the coffee is a perfect solution if the conversation gets awkward and silent.
I think you should tell your brother what you know, but don't tell him to overact and go mad - as I know most brothers do this as a protective sort of back fall.
So talk with her, communication is key.