Often writers of stories will delberatly make the characters missinterpret things in order to make characters more realistic (I mean how would they know evertything?) It is because of these seens I have assumed that all sorceresses are born that way, despite Rinoa's lines.

Not enough? Well I made an intepretation at the Edea parade scene, Rinoa takes an Odine Bangle to supress the Edea's powers, but when she does Edea makes her put it on her self. (That's how it looks to me anyway). This is because Edea is poseessed by Ultimecia at the time, and Ultimecia knows she will eventually posses Rinoa.

I believe that Sorceressess are always Sorceressess, but their powers are dormant up until a point, Ultimecia didn't want Rinoa to find out about her powers untl the right moment. She neeeded to get everyone up to the right point (I am not sure of that yet, but I think it was Adel).

This is just a theory, it needs more research, but feel free to respond, debate as well if you lik, I'll either retort, or add it to my DH list.