Sarah looked beautiful at Harry's wedding. Easily the best she's looked in 20 years. She clearly spent a lot of time figuring out the whole look and knocked it out of the park.
There's several reasons why this look works so well for her, and a lot of it comes down to mirroring the style of her own wedding dress in 1986:
1. Shoulder pads. Sarah's dress, like all 1980s dresses, had a major shoulder pad. The suit for Harry's wedding had a very clearly defined shoulder pad and it gives Sarah the posture and fit that she often doesn't have in her clothes.
2. Emphasis on the waist. Yes, the suit was bordering on too tight, but Sarah has a very well defined waist, so she was going to be damned if she let that go unnoticed. The same applied to her wedding dress - it cinched in her waist as tightly as possible.
3. Chest exposure. Sarah's 1986 wedding dress and the suit for Harry's wedding highlighted Sarah's chest, which is a good feature of hers that demonstrates the long neck she has, which often goes unnoticed.
4. Her hair was done properly. Sarah's red hair is a trademark and too often it looks messy. She should probably cut it given her penchant for just letting it fly, but she seems to love her long hair, so it's unlikely to happen. Her hair for Harry's wedding was perfect and seriously coiffed.
5. Makeup. Sarah often goes completely natural and it doesn't even look like she has moisturizer on her skin. Not this time. She was clearly wearing a lot of makeup given the state of Sarah's skin but whoever applied it did a marvellous job because it looked quite natural. It softened her features favourably.
On top of that, Sarah did not load up on jewels, wore simple shoes and had an excellent, but unfussy hat on. Bottom line: she looked like a Duchess and someone who was an HRH.
For Eugenie's wedding, I think she would look wonderful in a suit in the style Doria Ragland wore. Perhaps in a merlot colour. The A-line style is her friend given that her hips are the bane of her existence.