Weddings are normally a happy occasion, the start of a union and a cause for rejoicing, this marriage is a recipe for disaster and misery. If the husband is gay then the intimacy between husband and wife will either be forced or non existent. This will not be a happy day. If your sister fully understands the terms of this marriage then I feel sorry for her that she feels this is an acceptable choice. If she does not share your knowledge about her future husband then she is destined to be unhappy.
Unfortunately she is an adult and free to make her own choices even wrong ones. Ideally she doesn't need support on her wedding day but a reality check. We both know that will not happen. You family is so close to the situation they are limiting their perspective and choices by their understanding of the situation. JWs are very good at leading people by the nose and putting pressure to bear to make sure appearances are maintained. Your future brother in law probably made an issue of your attendance because he doesn't want anything to distract from his picture perfect JW wedding. I suspect that your sister did agonise but has to be submisive to the man who will be her head. This is the consequence of her bad choice. I think your sister will regret much of this and need your support again.
It is hard for you and painful because in all this your feelings have not been considered and you have been treated as a problem and not a person. Just remember that you are not a problem but have the potential to be a solution when she needs you.