Good for you taking the initiative to find your mom yourself with the help of your friend!
My thought is that it is very likely your mother became a drunk only AFTER they took you and your little brother away from her. That would be more than enough for most people to fall into despair and hopelessness.
I'm looking forward to hearing the rest of your investigation.
I had the same thought myself, I have an update which I am going to put on the end of this post.