Some time ago Square-Enix published that the theory behind Rinoa and Ultimecia is not true and that Rinoa and Ultimecia are not linked through time, family or anything else. The people of Square-Enix said that they are both different person.

I haven't seen this publish, but a friend of mine, his niece works at Square-Enix compagny said that this theory is rubish, it has even been published at Ultimania, that a sorceress has a human life span and that they live on and are weak as the humans. The difference between humans and sorceresses is that the sorceresses are stronger on magical niveau and dimensions, otherwise they are the same.

for more information regarding this official statement, visit here.