Seems like Mr. A has finally been tired of making up original plot twists, and not to mention character designs (*cough* Makie *cough*).
Negi X Asuna = Keitaro X Kanako (at least in the OVA and manga)
If I remember correctly (read: checked the scanlations), brother-sister relationships weren't supposed to be "official", that would be
incest, but some sort of comic relief...
Negi and Asuna aren't really official right, I mean, Negi has at least 28 girls (Eva, Chachamaru and Sayo no included) to choose from and hey, there's always room for a realllly twisted plot twist (double redundancy?)