From what those that really know titles and styles and all what needs to be known about them, I can pass on what I've learned.
When Sarah and Andrew married, his mother, the Queen created Andrew the Duke of York. By marriage to Andrew, just like it is still customary for a lot of women now, she took on her husband's "name" or rather title and styles of it. She was HRH The Duchess of York (and all of his secondary titles too). She was in that long list of titles also Princess Andrew as as stated, one takes the form of the males address. There's a lot on these forums that really get into specifics. Sarah and Andrew divorced. As an ex wife, she lost the HRH and is allowed to be styled as Sarah, Duchess of York. The style for those that know denotes a person no longer married to the Duke. It was the same with Diana. After her divorce from Charles, she was styled Diana, Princess of Wales. What most don't see is there's a difference. THE is the key word. Should Andrew remarry tomorrow, his wife will be The Duchess of York and all the feminine versions of the titles he holds. Sarah though could still use the style that she's got as Sarah, Duchess of York. The key point to me is that if you see the person's first name and a title (or style) after it, she's her own entity and not married to the title. To make things more clear on titles and styles.... Camilla is HRH THE Princess of Wales but is styled as HRH THE Duchess of Cornwall. Should they divorce, she could be styled as Camilla, Duchess of Cornwall or legally even as Camilla, Princess of Wales. Its confuzzling and I've
for ages trying to get it right. I had good teachers here.
To be honest, I don't think that if Sarah and Zara met each other in a "posh" boutique, they'd recognize each other. As far as the the princesses' marriages, Sarah will be afforded the highest honor of being mother of the bride which is what she is. How the seating goes, I don't even want to guess.