I got to say if he isn’t going to sort out his drinking for the sake of his partner and children then do you think you can sort this for him?
I have experience of this with my mother. She ended up homeless too (became violent), her brothers immediately had the attitude that, “right we’re going to fix your mum seeing as you couldn’t do it, we’re family we’ll sort it out.” It really hurt us that our uncles, our family too were rallying around my mother after she’d wrecked our home and our lives, leaving us in a deep depression and feeling like failures whilst being suicidal. Were we not family too and did we not deserve some support?
Within a year they soon realised like we did NOTHING could be done. Unless the alcoholic wants to do it for themselves. Of course my mother gave them all the responses they were looking for and looked like she was getting better, in truth it was just easier for her to hide it as they were so naïve.
Our story did not have a good ending, mother died and I’m estranged from my uncles.
Please think who in your family is the most in need of your love and support right now, is it your brother or his children? You say they have their own issues and I bet money on most of them stemming from your brother’s alcoholism.
At the very least be very sensitive in how you deal with the wider family.