No his application will be handled as an individual and not an immediate family member since he is over age 18. He'll either need another sponsor or he can apply for a student visa to study here. Your sister can sponsor him but not under a student visa and she'll have to line up a job like she did for you.

Your son will likely fail to qualify as eligible for immigration with you. There is no derivative status for family members with the exception of certain marraige based visas. Your husband will likely fail to qualify as well.

Since your son is studying in Europe, he may be able to continue studies in the U.S. or obtain a skilled visa after he graduates.

The sister is able to sponsor her sister with a relative based petition. It normally takes up to 12 years depending on the country. The sister does not have to line up a job. The child is an immediate family member because he is 21, but there is no derivative status.

Your son he must apply for him self for a visit to u s a and give your sister address as a proov were he go to stay and he cannot stay more then 3 month and he connot brack the law to stay in u s a that with the right thing to do as a bigenung in the first year (2) after you get the green card for 24 month and u have a living place for you and your husbend plus u have proof of income thin you will have the right to apply for your son to come to u s a legaly i wish u good luck and i hope i give you the answer you need good luck to you and welcome to the great country of u s a

You do not have to wait 2 years to petition your son. Once you arrive in the U.S., you can petition him right away. The F2 category for many countries is backlogged 10+ years for unmarried children over 21, so you may be in for a long wait.

When you say Approval Letter, did you mean that the petition was just approved by the USCIS? or it is a letter from the NVC?

With regards to your son, albeit he is already 21years old, he "might" still be qualified to benefit as a derivative beneficiary of the petition. But it has to be assessed first. There is a so called Child Status Protection Act (CSPA) that gives remedy to such situation or cases as your son. Consequently, the provision and the process is quite complex. You may refer it to a qualified immigration consultant to ascertain the eligibility of your son to benefit the CSPA.

I doubt that under the CSPA it would make a difference- since we are talking about 2-3 weeks time before the 21st birthday.. Your son will require his own petition and will NOT be able to get derivative status through your petition. Since the I-130 has just been approved, the backlog is so great that you are looking at a MINIMUM of 10 years (for every nationality except Mexican or Philipino) before you can emigrate to the U.S.. If Mexican, the wait time is around 17-19 years. If you are Philipino, the waitime is around 21-23 years.
As the beneficiary sibling of a U.S. citizen, you are in the family based petition- 4th category. You can look at this information yourself by looking up "visa bulletin, January 2011".