OP, I can understand you being upset that your DH ended up drinking after it being suggested he go to the pub with BIL/SIL. They get to see the initial stages of him drinking again, and probably far from the worst of it, but you (& kids?) get to deal with the fallout.
The stress from thinking about this alone must be very difficult for you. It's not a bad suggestion to try Al-Anon. I grew up with an alcoholic father and a mother who would say he should keep away from people who "would influence him to drink". But that didn't work.
When I look back now, I think he needed to change his attitude to alcohol completely, he saw it as something that "they" took away from him, but actually in his case he was using it to deal with emotional pain from a neglected childhood.
Coming to terms with being an alcoholic is about far more than avoiding the pub. Yes, at this stage he probably should avoid the pub, but on it's own that's not enough to help him develop a new life without alcohol.
When I think back to my childhood, I believe my father's siblings had no idea what he was like when drunk, having left his "friends" in the pub and arriving home to create utter havoc. If they truly had understood, they surely couldn't have encouraged him to drink.
It could be that your BIL and SIL don't actually have a true picture of what you have had to deal with, and that's something that's hard to get across to someone who doesn't witness it as closely as you do. The nature of alcoholism, for some reason it's something people keep as secret even amongst those who "know", it's just touched on, "Johnny's little problem, better keep away from temptation", but actually what people need to know is how you and your family are affected. Alcoholism, and abuse generally, thrive in secrecy.
It's shit, you didn't invite this into your life, but being open and upfront about it is your best bet in dealing with it. Minimising it, or, as in my family's case, sweeping it under the carpet, does not work. In fact it only stores up more future pain for too many people.
As others have said, it's your DH's choice if he drinks or not. But it is common sense not to go into an environment where he has spent time being comfortable with alcohol. Could your BIL/SIL come to you - I realise you may feel bridges have been burned, but is there a possibility of rebuilding?
Is your DH getting support such as in AA? If not, why not? This is not something to deal with by oneself. It makes the partner, you in this case, "responsible" for helping the alcoholic. You both need a wider support network. And honesty. Good luck.