F-4 Visa for childrens who are over 21

My sister's F4 application was approved 1997 and she is due to go for her interview any day. My question is - I was recently made aware that although at the time the application was made and approved her 2 kids were under 21, but now 12yrs later they are both over 21. What is the next step to take. I was told she has to petion them onece she is in US and the priority date will be from 1997. Is that true? How does that work?