what a PITA.
The problem is that she's going to be family for life so maybe a compromise might work. I personlly don't like the thing where all the bridesmaids wear the same dress because it always looks dreaful on one of them who has the wrong body to carry it off. Maybe you could just pick a fabric and let all your bridesmaids choose a style that they like.
As for your hen's night - too bad. If she doesn't like it then she doesn't have to go.