Adopting through foster care in the USA will also vary greatly state by state. If you're looking for a sibling set then you most likely will end up going international or via foster care.
My husband and I went through the entire process to be certified to be foster parents and then eligible to adopt. Though for us, it was because we had 1 month old "relative" in foster care and we wanted him placed w/ us. Also to be eligible to adopt him if he was not to be returned home. We had to go through both of the processes because we were not a "legal-blood" relative. It was my MIL's long term boyfriends son's baby. She's been w/ this man for about 15 yrs and she's raised his son in her home since he was 7-- so they seemed like family-- but the "law" doesn't see it that way. So we were given some special considerations like an expedited application and such, but we still had to go through the process.
Anyhow, when my nephew was returned home at the last second (and my heart was breaking missing him) we thought about fostering to adopt. We spoke w/ the case workers about it. In our area it is really difficult as most kids do not become eligible to adopt in our area. To adopt out of the county seemed even harder. So we didn't pursue it any further.
However, in many other states it's a lot easier from what I've read.