I think you're afraid to be honest with your husband about your feelings, which I understand.
You tried giving him an out, telling him he could stay home, but he didn't take it. That is unfortunate. He could have agreed to staying home and thinking it was his choice. That would have been easy.
Pick the right time and tell him the truth, gently, but seriously.
Tell him honestly that YOU are extremely stressed out by all of his horrid comments about the wedding. And YOU are VERY concerned something at this event will infuriate him, and his true feelings about your sister will be obvious enough to make YOU and OTHERS miserable. Acknowledge that your past venting has contributed to getting him riled up, but that you WERE just venting. Tell him that YOU genuinely love your sister and family and you REALLY want enjoy this special event with them.
Then just wait. Pause. He doesn't have to respond right away. Give him some time and quiet space to respond. His response will be very telling. His response will tell you if is truly cares about your feelings and about other people. Or if he will disregard your feelings and make it about himself. If he cares about YOU, he will see how his behavior is effecting YOU, and he will stop making horrid comments, attend with you and be perfect gentleman even if he despises your sister, because he should be able to act like a grown up and a decent husband. Alternatively, he should acknowledge his own stress triggers and anger issues, and not attend, so as not to ruin the day for you and your family.
If he still insists on coming along with huge chip on his shoulder? To me that says he's not willing to make an effort for YOU. And you're going to spend the whole event taking it on as your responsibility to placate him. That isn't right.
I know you're afraid of permanent repercussions, but if you haven't told your husband how worried you are about all of this, you aren't being honest with him. You're hiding your anxiety, and he's clueless. You really have to lay all of your cards on the table, and then take a good honest look at what your husband does in response.
Right in the moment you tell him, he may quickly blurt out something defensive, but if you stay calm, don't engage, and give him some space. It may just sink in, he may come around and realize that he needs to change his behavior.