their ages, their backstories, their graphically represented appearances, and the full extent of their personalities
1. Age is more easily disguised than skin color.
2. We have a long backstory on Briala. In particular, she was raised in Orlais and almost certainly has an Orlesian accent, and she's all about sacrificing the few for the good of many. We know Sera's accent, and we know she would be violently opposed to everything Briala did and what she stood for.
3. Briala is described as having tan to dark skin. Sera has very pale skin. Consider for a moment the (justified) uproar that would result if Bioware said "lol nope, we decided to give Briala light skin in-game."
4. Briala is about the long game, the big picture. Sera is about "what's right
now." Like many of the city elves Briala came in contact with, Sera thinks those in power are pitifully out of touch. That's not the opinion of someone who
holds the key to the entire network of Eluvians.
In short, Alistair is more alike to Loghain than Sera is to Briala, even based on our limited knowledge of Sera.