Add my name to the list of childless 20-something Disney Cruisers (or DINKs, as we like to call ourselves... dual income, no kids). Leina and I took our first cruise on
DCL when we were 22. We weren't honeymooning, weren't tagging along with anyone, we just went because we wanted to take a cruise vacation on our own... and it was the most relaxing vacation we've ever taken.
We are 27 now, looking forward to our second. Some things to keep in mind though:
1) Disney is by no means a 'party boat' when it comes to adult couples. It's a pretty mellow cruise ship. If they're looking to party all week long at all hours of the night, they will be disappointed. If they are going to relax, be pampered, enjoy the activities, and enjoy the ports, then they will do fine.
2) Chances are that there will only be a handful of OTHER childless 20-somethings on the ship. There is a little bit of a demographic divide, it seems. There are quite a few older childless couples. There are quite a few younger couples with kids. There are a lot of kids and teens. But there were only a handful of kid-less young adults. On the up side, Disney appears to try and group all these folks together during dinner rotation. We were with another 20-something kidless couple, and the tables around us were mostly 30- and 40- something adult couples/groups.
3) If they want to be away from the kids, they CAN be away from the kids. We didn't realize how many kids were actually on the boat until, on the LAST day of the cruise, we happened to walk past the line of kids getting ready to participate in their end-of-cruise event (talent show?) in the main theatre. We said to each other at the same time 'holy cow, where did all these kids come from?'
4) They will not be 'Disneyed to death.' Everything does kind of have a Disney air about it, but it's not overwhelming.
5) There will definately be clubs and other kidless stuff for them to do in the ports, if they are so inclined.
Depending on their personality, they MAY like the party atmosphere of Carnival better... they will have more folks their age w/out kids around... but your siblings should still have a very relaxing and enjoyable vacation on DCL.
Brian