Okay, if a Sorceress needs to pass her powers on before she dies then in theory Ultimecia really is a descendent of Rinoa.
Don't flip out yet!
So waaaaay back when there was Hyne, she created the world blah blah. So many many many years later we have 2 sorceresses existing, Edea and Adel.
Before them there were many sorceresses made by Hyne (there had to be, or else how could we have 2 sorceresses in one time period?) but they were hunted etc. and eventually there were only 2 sorceress bloodlines, the one that eventually passed its powers onto Edea, and the other to Adel. Now, because Adel died it eliminated the second to last sorceress blood line (well, its not literally a bloodline, but you get the point) and that left Rinoa, whom had recieved her powers from Edea. So now there is one sorceress in the present day, Rinoa.
So Rinoa MUST have passed her powers on before dying, lets say to some girl named Lenne, then to Nina, Eleanor, Kelly, Nadia etc. etc. so all these sorceresses (yeah i know i made them up, but go with me) passed down their powers to the next, eventually getting to Ultimecia, so really Ultimecia has to be a descendent of Rinoa, but they definitely aren't the same person.