This makes absolutely no sense. Neither DA'd/DF'd but still not allowed to participate in the wedding party? And, if the wedding is held in a neutral place, does this mean the r&f won't attend? This is horse crap for sure. I'm sure its extremely difficult for your parents & sister right now, and I hope everything works out.
Weddings are one of the happier events in the congregations, and to turn it into the situation it has become will probably make alot within this particular congregation go "huh"? There might be a silver lining, I'm just sorry its at the expense of your sister, soon to be brother in law and the rest of your families happiness.