Aside from the fact once the phone is powered down and literally a brick.
Aside from the fact that Ya Kun is nice enough to share the footage.
Believe next steps should be to lodge a police report?
Only the police would have the authority to request for CCTV data from Ya Kun as a formality for investigation.
Owner should be able to retrieve S/N or IMEI as part of the reporting.
So if the phone is indeed sold 2nd hand, there is literally a database to reference against.
Then auntie might get in trouble trying to sell it.
Otherwise i honestly don't think she will have the smarts to jailbreak it for useability.
It might have benefited her more just to turn it in to Ya Kun.