It was Snowdon&Chatto type of events. It wasn't Catherine's type of events. Look at other guests.That's great, but it's not Sarah's dress is it. I never said it didn't make sense that Sarah attended either.
Muriel said Catherine doesn't wear a lot of Dior which seemed to me like a need to attend this exhibit, as far as I'm aware neither does Sarah.
Also having Princess Margaret's dress on display, should be another reason Catherine could have gone.