Your sister had the same exact experience that I had last Friday. My file was also kept for additional review. They handed me a slip in which was stated that they have all the required documentation and will reach out if they identify anything missing. There is no hold up in the system (DV file, AOS package and bakcground checks are all cleared). They also know the case is time sensitive.
I have been "stalking" them with phone calls and also requested another expedite request on case adjudication. Today someone from FO called back and asked me to give it time until next Wednesday.
That has been my experience and I have no idea why they are doing it in fact. I would like to think that there are taking their time.
What I have done so far and what she can also do are:
Set 2 infopass appointments (you can do the second one by switching the last name as first name
)
If you don't see the status updated within 5 days, raise another expedite request for case adjudication with the National Service Center.
Get engaged with your congressmen local office. Ask them to ask whether the process has bottlenecked ?