MikiMo

I had the same scenario,  if you mailed the right documents,  wait for one week

Not 100% sure since this is kind of a unique situation - if my math is correct your father filed for her 12-13 years after her mother passed away? 
 
But typically a marriage certificate + step-daughter birth certificate should be enough to link the step-father/step-daughter. I would bring anything else that could possibly be useful.

Nope and the unmarried kids under 21 get to tag along on the F1.
Moreover: "Unmarried" for purposes of F1 means a few things:
Single, never married Divorced Widowed Marriage annulled

Thank you all for the responses. 

Then they should be ok. 

Not aware of any kind of certificates of no marriages being available anywhere in Africa. I am assuming though there has also been no traditional marriage? 

That I am not sure though. You may want to check Your embassy to be sure. I apologize I generalized my statement. In the Philippines, we are required to present the certificate to prove that we were never married. You may want to check the travel.state.gov for country specific civil docs requirement. Please see attachment. 

Most countries do not have a “certificate of no marriage” FYI, so no it is not a requirement for remaining in this category. I know it’s a big thing in the Philippines but it’s not clear where OP is from. 

If there has been no marriage the son/daughter still qualifies as F1, and in fact can include the children as derivatives on the petition. The son/daughter will need to stay unmarried until he/she enters the US on their immigrant visa and becomes a LPR. 

Hi! You mean by never been officially married, but living together? I think so. Your category remains as long as you can provide certificate of no marriage. It’s okay to have children and even add them as derivatives, as long as you are not married.

We talked to NVC and they told us the technical team is still working on the issue and can confirm by looking at some minor change in the account. Also got a call from NVC a few days ago asking me to try to make the payment again though could not proceed because the AOS payment status had moved from Start to IN PROCESS and has been like that for 3 days. We will call NVC again and keep you guys posted whenever the issue is fixed.
 
The best advise for now is to use the online public inquiry form, get snapshots of the error and also explain in detail the steps you are taking and the error message you get. Try all browsers and make sure to mention that in the public inquiry request you submit

This is really very unfortunate. We've in the same boat as you for the last 7 months. It's really very unfortunate that such an issue can happen in the US with such advanced technology and smart people. Our issue has been making fees to NVC. Considering that you are having the same issue with USCIS, it's starting to sound fishy what exactly is going on. My relative has had to call NVC multiple times and keep getting scripted responses what we need to do, being told to do the same thing over and over by different agents. Claiming there are no screen shots of the error submitted before yet they've been submitted multiple times. We are now trying to come up with a next step because this is ridiculous. Sadly enough, they don't accept any other form of payment other than the online payment that's not working.