Mutahir

I have observed it multiple times as well. To my understanding/guess so far, case worker looked at your case on Saturday/Sunday for one reason or the other.

Assuming that your niece was interviewed along with your sister, there is no need to worry about it.  Her age is locked when the your sister's case became current/interviewed.
 
CSPA is automatically calculated first by NVC and then by the Embassy.  Final judgement is made by CO though.  Only my sister received interview letter from the Islamabad Embassy but we advised her to take all her kids (over 21 years of age) and get the CSPA calculations done at the Embassy.  3 of her kids were able to get visa while the oldest didn't get it as she didn't qualify for CSPA. 
 
Good Luck!
 
My younger sister is under CSPA since April 4th 2017.
 

Sorry, she is under AP since April 4th 2018.  Typing error.

Assuming that your niece was interviewed along with your sister, there is no need to worry about it.  Her age is locked when the your sister's case became current/interviewed.
 
CSPA is automatically calculated first by NVC and then by the Embassy.  Final judgement is made by CO though.  Only my sister received interview letter from the Islamabad Embassy but we advised her to take all her kids (over 21 years of age) and get the CSPA calculations done at the Embassy.  3 of her kids were able to get visa while the oldest didn't get it as she didn't qualify for CSPA. 
 
Good Luck!
 
My younger sister is under CSPA since April 4th 2017.
 

Case was updated yesterday. Just the date changed. 

Pay the fees quickly.  Once the fees is cleared they will open your Online IV application.  NVC will not review your documents if your online IV application is not complete.
 
You don't need police certificate for NVC.  Police Certificate is needed at the time of Interview as far as I recall from my sister's example.
NVC will take about 7-10 weeks to respond once they receive your documents.

After I-130 is approved, case is sent to National Visa Center, NVC.
 
As soon as the NVC receives a case they assign a case number to it and will inform you with the case number and prehaps next steps.  NVC can take 60 or more days to notify you of your NVC case number.
 
Steps to follow will be explained in the Welcome Letter from NVC once the application is ready for further processing.  Expect to pay $120 + $325 in the next steps with NVC and fill out DS-260 for your mother

If you don't hear from them in 45-74=5 days or so, don't hesitate to call NVC @1-603-334-0700 to find more details from them.  
They will ask USCIS receipt number, name of the petitioner, etc.

Existing application cannot be eliminated.  However, they will most likely stop taking F4 applications at some point in future.

After I-130 is approved, case is sent to National Visa Center, NVC.
 
As soon as the NVC receives a case they assign a case number to it and will inform you with the case number and prehaps next steps.  NVC can take 60 or more days to notify you of your NVC case number.
 
Steps to follow will be explained in the Welcome Letter from NVC once the application is ready for further processing.  Expect to pay $120 + $325 in the next steps with NVC and fill out DS-260 for your mother

If you don't hear from them in 45-74=5 days or so, don't hesitate to call NVC @1-603-334-0700 to find more details from them.  
They will ask USCIS receipt number, name of the petitioner, etc.

If you don't hear from them in 75 days or so, don't hesitate to call NVC @1-603-334-0700 to find more details from them.  They will ask USCIS receipt number, name of the petitioner, etc.

After I-130 is approved, case is sent to National Visa Center, NVC.
 
As soon as the NVC receives a case they assign a case number to it and will inform you with the case number and prehaps next steps.  NVC can take 60 or more days to notify you of your NVC case number.
 
Steps to follow will be explained in the Welcome Letter from NVC once the application is ready for further processing.  Expect to pay $120 + $325 in the next steps with NVC and fill out DS-260 for the 14 year old son.

My Sister's kids name are misspelled in the CEAC web site.  Do we need to do anything or make corrections when filing form DS-260.

A. This is a chart for interviews when the Priority dates becomes Active.  Interview Scheduling...
 
B. This chart tells that you can start filling our Visa Applications.  If your priority date appears or fall within the dates mentioned in this chart, you should expect to receive a Welcome Letter from NVC.  Welcome letter tells you what to do in 6 steps.

The way I understood from reading several web sites, I come to conclusion that ff they do eliminate F3 and F4 categories, it will be for future date. They will stop taking applications for F3 and F4 starting from whatever date they decide, for example January 01, 2014.
Existing F3 and F4 applicants will continue with their application process until Immigration Visa is granted.
Let's wait and see.

Please be patient and stop pinging again and again.
When any one has any update, you will see it.

Yes i suppose they will because most of us act at the 11th hour. So there will be many applicants for AOS till 30th April 2001. I-245 law allowed for illegal immigrants to be petitioned for after they paid $1000 fine and were physically present in the US on 21st December 2000. So after we get over April 2001, dates should start moving at a better rate. Ofcourse i cld be proven wrong in less than a month when the next vb cmes out, but thats jst my take.
And it was jst common sense, if theyre not sending out ILs then it was sure the dates werent moving far. And one thing i learnd following my case in the last 3 yrs is that absolutely no one can predict the VB. They take your hopes to the sky only to smash them into pieces, multiple times!