Hi,
I'm sorry for the situation that you are living in.
I hope that your boyfriend's brother finds another place to live. Clearly, his being there, is not going to be good for your relationship with your boyfriend. You can get sober without his support, but I think he will need to accept that your lifestyle is changing.
And, try to understand that it's very hard for others to know addiction, and how we can't stop drinking, once we start. You know this, so you need to do whatever it takes to stay sober.