As a sister, I'd like to have Osaka from Azumanga Daioh. She's so sweet, but is very funny in how she's always so spacey. And I'd love to sit and listen to her strange new hypothesis of the day. ^_^
...as opposed to your current sister, who is more like Osaku crossed with Tomo (tomo? dangit who's the hyper competitive one?)
No I'd like to have my older brother be Kaji from Evangelion. I think he'd be fun and would have been able to teach me awesome stuff.
If I had a little sister, I think I'd want Chiyo from Azumanga Daioh. She'd be adorable and I'd use to pick up chicks. Heh heh.
Also she could help me with my homework!
Also, my dad would then be AWESOME!