I am sorry I am probably not qualified to comment on this thread because I am from the 'other' side, but your situation makes me feel mad and sad.
Why should you and your boyfriend have to move out?
Why should you have to leave your mum?
Why can't your brother move out and spread his misery elsewhere?
I am glad you are here, but I am sorry for the circumstances that brought you.
I hope you continue to come and get the wise advice of the experts here.
Remember too, we have al-anon in the UK.
In time perhaps your mum might come with you.
My best to you x