Here's what I think.
Arika is(/will be) Negi's mother and the wife of Nagi. Asuka is the sister of Arika and if this is the case then Negi and Asuka are actually related as a nephew and aunt. This would expain why Negi and Asuka have the same bangs and possibly her protective instincts towards Negi (with him being the son of her older sister.)
Also this would explain why Asuka looks so much like Negi's sister. Asuka would be her aunt as well. Don't ask me how Asuka appears younger then her, but we have already seen some timetraveling/suspension in Negima.
It could also be that Negi's sister is actually his mother in disguise. We have seen some appearance-alteration in the series as well so who knows? In this case it would explain why Negi's sister appears older then Asuka. :P
It is just a few things I came up with. Maybe some others would like to share how they feel on this? ^^
By living outside the circle of destiny, one attains true freedom of creation...