This topic has been discussed few times on this board.
the parents will get their GCs because they are immediate relatives of USC that is not subject to visa preference.
the siblings have to go through visa preference category through their GC parents or USC sibling no matter their ages are over 21 or under 21 because immediate relatives of USC cannot bring their children as derivatives.
If I recall correctly,The derivative status only applys to preference immigrants and asylum applicants because principle beneficiary has waited for visa number for years and K1's under-aged children (K2 status) when K1 marrys to a USC
The other poster asked what happens if beneficiary turns into 21 after his parent file I-130 under F2A
Assume your parents will file petition after they receive their GCs
The beneficiary category will automatically change to F2B (unmarried son or daughter of LPR) if parent stays as PR or notify USCIS to change category to F1 or F3 after the petitioner naturalizes in 5 years. The original priority date will be retained, but wait time will vary based upon beneficary's marital status and petitioner's immigration status.
Here is what the laws say: read first sentence carefully under follow to joint benfits:
Following-to-Join Benefits
Please note: This section is only applicable to lawful permanent residents who did not gain their LPR status as the immediate relative of a U.S. citizen.
Minor's (your sibling) relationship to an immediate relatives (your parents) of USC is not on the list
follow to joint benefits:
http://uscis.gov/graphics/howdoi/childproc.htm#following
Good luck