Best way to deal with it
I have already written few times about this and I will do it again. The best way to deal with it would be to let parents become GC and apply for their son under F2A then automatically convert to F2B (Unmarried son or daughter of LPR over 21) after the son's ago over 21. In 5 years, the parents apply for U.S. citizen, then upgrade the case to FB-1 or FB-3 and retain F2A/F2B priority date based up the son's marital status. Here is the logic behind:
GC-USC=5 years
USC-FB-1 wait time=4 TO 5 years
Total wait time= 10 years
GC-F2B wait time = 10 TO 11 years
GC-USC wait time =5years then upgrade, most likely the son will receive greed card in 6 years if he is single at the time. He potentially save 4 years waiting time.
In addition, let the sister apply him under F4 in case of parents's circumstances changed in the next 5 years. You can file multiple petitions from different petitioners and category. the worst scenario would be to start overall again. You may spend a few hundred dallars initially, but you are well protected. So you should have three petitions from the father, mother and sister. You must do this if you have aging parents. GOOD LUCK