Remember when Spyke was Ororo's nephew, even though they still had Storm grow up in Africa as a goddess(presumably after being orphaned) and yet they never bothered to explain how Ororo had a sister(who was apparently American)?
I didn't think this needed to be explained. Ororo was lost and thought dead or something while her parents were on vacation and her older sister was taken in by some other living relatives.
"We come into this world alone and we leave the same way. The time we spent in between - time spent alive, sharing, learning together... is all that makes life worth living." - Jean Grey