1) Seifer copied Dad's (Laguna) attacking style in his sword form.

2) Seifer and Squall both use swords. They both have facial scars. This proves that thier genes were designed in the same way by their parents.

3) Seifer and Squall dislike each other for selfish reasons. About the same dislikement Squall has for Ultimecia. Ironic, eh? Quite common in brotherships. Think about it, Zell is annoying just like Seifer, but yet Squall hates Seifer more.

4) Seifer and Rinoa were in a relationship. This proves that Squall and Seifer have the same taste for women.

5) Seifer had a childhood dream, who says Squall didn't? Squall was shy and insecure, so of course he won't express himself fully...

6) Squall and Seifer both have similar facials; thier mouths, eyes, jaws, and eyebrows. Except thier hairs... but I believe that Seifer dyed his hair blonde (yes, even in his youthhood!)

7) There may be some preposterous claims made by Kiros/Ward that Squall looks like Laguna. But if he mentioned Seifer, it wouldn't make sense, right? Why would a friend mention an enemy's name when it's relevant? We beaten Seifer.. and go like "You and Seifer just look like Laguna..." Anyways, if there's no hardcore evidence within the game that Squall is actually Laguna's son, then the same goes for Seifer. The concept of Squall being Laguna's son was born from a hint. Keyword: hint!

8) The last names of Seifer and Squall may not match, but who says one can't change his name? As far as I know, it's legal to change your name under the age of 18... in a game such as VIII..

Hmm, interesting. I sometimes wondered why Ultimecia chose to use Seifer as her main puppet. I mean, why didn't Ultimecia possess someone bigger like Ward? Imagine beating a possessed Ward..
But consider the acknowledgement of a brothership conflict. I believe that that would help Ultimecia succeed effectively... far more effective than possessing a guy like Ward.

Uh huh, Ultimecia didn't possess Seifer because he was confused in his unpossessed form.