There is nothing you can do other than to accept and be there to pick up the pieces when it happens.
You can have a quiet word with your brother when he gets back and say your husband has told you what he said "in drink" whilst they were away.
Just tell him you won't tell him what to do. However, you will support him if he chooses to leave her before the wedding or anytime in the future.
There is a chance he will deny it and will say, "It was the drink talking, and it isn't like that." Try not to put his partner down when you talk with him about it as doing so can put him on the defensive.
Long-term, it doesn't bode well. I'm a survivor of domestic abuse. Even though I'm female, I acknowledge it happens to men, too.
I'm not sure where you are in the country. However, a section of Womens Aid charities in each area became IDAS (Independent Domestic Abuse Services). The areas that have these mean they also deal with men.
Even if there isn't an office in your area, ring the H.Q, and they can advise.
Let him know what's available and that you support him. Whatever you do, if he decides to stay, you will need to find a way to be civil to her for his sake.
It's a complex situation with abuse. I hope the little I have written has been of help.