Just as there is nothing you can do to change or "fix" your brother, there is nothing you can do to fix your mother because she doesn't think she's broken.
So...what to do.....aetheist or not, there are some very useful tools you can use from the various 12 step programs. The first of which is "take what you need and leave the rest". So take some of the tools and adapt them to your beliefs. There are quite a few atheists in these programs believe it or not.
Instead of "let go and let God" you can use "let go or be dragged" for instance.
The serenity prayer can simply be an affirmation statement to yourself rather than a prayer by eliminating the word God....speak as though you are speaking to yourself or a dear friend. Or change it up a bit like......find the serenity to accept the people I cannot change, the courage to change the people I can, and the wisdom to know its me.
It all boils down to self care. Taking care of one's self FIRST is best. Not in a narcissistic way without care or thought of others but in a self loving manner. If its not good for you to be around your brother....make no excuses and do what's best for you.
Life changes. Things change. And people change but not because we convince them to do so. It's the events around them that change them. You have a right to state your boundaries clearly to your mother. A great example of a boundary in your case is "I will not go anywhere where pot is being used.". There is nothing mean or angry about that and you shouldn't feel obligated to say anything more. It's pretty clear and it's about YOU. Not about your brother and not about your mother. Another boundary "I will not listen to conversation that revolves around brother. I will terminate the conversation if it heads that direction.". Again....nothing mean about it. Its about you. Say what you mean (state your boundary clearly), mean what you say ( boundaries are not negotiable) and don't say it mean (be nice about it). If someone has a problem with that, they own the problem....not you.
Congrats on getting pregnant after so much difficulty. You have wonderful things ahead as you grow that little one in your womb and prepare for birth. You'll be way too busy to participate in the dance of addiction.
By the way, I'm the mother of an addict and nearly lost the relationship with my daughter due to my son's addiction. She detached from her brother and began vehemently detaching from my codependent nonsense. I changed to preserve that relationship with my daughter. She's every bit as important to me as my son. But it took a while for me to realize what was happening. I am very grateful that she took a stand for herself.
Bottomline here......take care of you.
gentle hugs
ke