It's better for them to be siblings. I just don't see akai and shiho relationship working out lol. Can't even imagine it since they're both too cold lol. Shiho's perfect match is Shinichi, really. Shinichi's kiddish behaviour just fits with Haibara's cold personality. He's the only who can bring out humor from her lol
And as I said, I feel like no character right now fits the bill for shiho except shinichi. Aoyama needs to introduce someone who's personality is just almost the same as with Shinichi or Kid, If he wants to sway her feelings for Kudo away.
As for Akai, Jodie complements his personality. We're also forgetting this. Jodie has been also waiting for Akai for too long.
Anyway, who's Zero?