Is she the executrix? If she is, she has no right to hold up division of the terms of the will and make the dispersal of the money/property contingient on whether your DH shows up or not. I believe that most states have a lawyer acting with an executor/trix, so it might be in his best interest to contact the lawyer and ask that his inheritance be sent to him by mail. (This is if he can't convince his sis that getting to the meeting she wants is just impossible - because of work obligations, illness, some kind of excuse that can put off this meeting indefinately).
However, if his father did not leave a will, or the sis is otherwise just informally elected to decide how best to distribute the inheritances, then he might have to visit in order to get his share.
Soooo. The big question is: Does he feel that getting the money will outweigh the discomfort and opening of raw wounds that the meeting will most likely cause?
And what would happen if he just told his sister the truth - that he is very uncomfortable about the idea of meeting with all of them and drinking, because he does not want to get into fights or bring up bad feelings right now? If she is fine on her own, then she might be receptive to listening to him and understanding the meeting is a bad idea.
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My DH cut off his parents almost two years ago. I asked him how he felt if they wrote him out of their will, and left all of the family heirlooms to someone else, and he was fine with it. In his family, material things (like the antiques, etc...) were more important than the people. His mother obsessed over a few items and pieces of furniture (that were just old, not worth much) that belonged to her inlaws, and when they died, she carried polaroid photos of the items in her purse, and whipped them out to show strangers, much like a proud grandparent might do with pics of her grandchildren. She also would say in a gloating and scary voice "AND THEY ARE ALL MINE!" DH's grandfather had told both of DH's parents that certain things should be given to DH, but it wasn't written in the will, and DH's parents didn't pass anything that they were told to give him on to him - they decided that he would get it all when they died, so why give it to him now?
DH decided that while he would have liked items of sentimental value from his grandparents, that the cost of inheriting them would be to continue putting up with his absolutely awful parents and their treatment of him. He chose to cut them off, hurting himself as far as sentimental objects and fiinancially, but saving himself years of heartache and pain from bitter, twisted people who are most likely narcissists to boot.
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So, ultimately, your DH has to decide if the pain of the meeting is worth it to him - and I don't mean just financially. I don't know what your situation is, but if the money is needed and was promised to him, then maybe going and getting it over with won't be so bad if he just refuses to be baited into arguing (difficult, I know!). I am on the fence about you going with or not going. I feel like it might be good for you to be there to be supportive and might discourage the brothers from acting as bad as they could, (although they may anyway if they are used to you) but I can see where it would be very uncomfortable for you as well.
If it was me, I'd try to talk to the sister and get his part settled without involving the brothers.
It sounds like a situation made even more complicated than it needed to be - you and your DH have my sympathies.
