- You may be looking for Tegan Jovanka's aunt.
Vanessa Prior was, as far as the world knew, the daughter of Aubrey Prior. In fact, she was a clone of the Nephthys-imprisoning body of Sadan Rassul's daughter. Prior created the clone after coming under Rassul's influence.
Unknowing of her true identity, she got engaged to James Norris. In 1996, though, her mind was finally engulfed by Nephthys instinctive half. When Nyssa finally awakened, Vanessa/Nephthys did not find her intellect side in her, only an old woman. This was because the Fifth Doctor tricked her, and substituted Nyssa's double, Lady Ann Cranleigh, at the age of 90. The Doctor convinced her that Nyssa had been in a trance since 1926, and Nephthys' intellect had dissipated in the intervening time. Vanessa/Nephthys travelled back to 1926, only to find Nyssa still in a coma. She returned to 1996 to verify her intellect was still not in Nyssa, which it wasn't. Without her reasoning to guide her, Vanessa/Nephthys looped between 1926 and 1996, ageing 70 years each time. Even though extremely longlived, Osirians were not immortal, and she eventually died of old age. (PROSE: The Sands of Time)