Now I've watched a lot of Ranma 1/2 and a lot of El Hazard, and I've noticed one thing about these series: Each has a mostly insane brother/sister combo. Upon further examination, I noted that one of the siblings was always completly wacko, while the other was a lovesick fool. And when I started noticing similarities in appearances, I came to one conclusion:

Someone mixed them up at birth.

Exhibit A

Kuno and Nanami

Nanami Jinnai	Tatewaki Kuno
Is in love with a boy who must dress up as a girl.	Loves a girl who he doesn't know is really a boy.
Addresses Makoto, her long-time opposite gender friend, as Makoto-chan.	Is addressed by Nabiki, his long-time opposite gender friend, as Kuno-chan.
Has a thing for wussy guys.	Has a thing for macho girls.
Is pursued by a cutesy girl who tries constantly to win her affections (Alielle).	Is pursued by a cutesy girl who tries constantly to win his affections (Mariko).
She's been chasing after Makoto longer than any of the others.	He's been chasing after Akane longer than any of the others.
She still won't get him.	He still won't get her.

Still not convinced? Then take a look at:

Exhibit B

Kodachi and Jinnai

Kodachi Kuno	Katsuhito Jinnai
Is completly, totally, insane.	Is completly, totally, insane.
Will stop at nothing to get rid of that evil Pig-tailed girl.	Will stop at nothing to get rid of that evil Makoto Mizuhara.
Why the above? Because the Pig-tailed girl is trying to steal Ranma, the man she is destined to marry (yeah, right).	Why the above? Because Makoto Mizuhara is trying to steal El Hazard, the world he is destined to conquer (yeah, right).
"OH HO HO HO HO HO HO!!!"	"BWA HA HA HA HA HA HA!!!"
Akane always manages to beat her.	Makoto always manages to beat him.
Sasuke.	Bugrom.
Was the champion of Martial Arts Rythmic Gymnastics, but only because she cheated.	Was the president of student council, but only because he cheated.