You are her great uncle or great aunt.
i am Elmo zumwalts great niece. i met him once when i was very little. my dads name is Kurt zumwalt. he jas three sisters named alise, jenean, and marta. he has one b rother named Eric. he is my favorite uncle(and my only one).im not sure if he has anyother brother(Elmo zumwalt) but im his greatniece. Reilly Zumwalt!
The terms grand niece and great niece mean the same thing, the granddaughter of your brother or sister. Some regions, and some people, prefer one term and some prefer the other, but both are correct and understandable terms.