Graceless (Big Finish series)

Graceless features sisters Abby (previously Amy, changed due to the introduction of Amy Pond to the TV series) and Zara; human "tracers" created by celestial beings The Grace, creators of the Key to Time. Introduced in the Key2Time trilogy (The Judgement of Isskar, The Destroyer of Delights, The Chaos Pool), Abby and Zara are now free from The Grace, and must learn to fend for themselves in our universe.

...
Wikipedia