We have done both Dream and Oasis 2x and last time DS was 12 1/2 and was in the tween section of both (cruised the same year). RCL also splits teens like
DCL does. Can't remember the names of the two, but the tweens club is 12-14 I think and then teens is 14-17. Hands down, DCL does the kids clubs better, IMO. My son rarely went to the RCL club, but he was alone (DD was 9 and in younger group) and he was at the younger side of things. That being said, we all (kids included) agree that we wouldn't do a DCL cruise for more than 3-4 nights bc even with the kids clubs being more fun, there is SO much more to do on Oasis. I think your girls may have fun in RCL club (I think girls have an easier time meeting people anyway and they'll be together) but even if they don't, there's so much for them to do on their own around the ship. I think that's part of why the RCL older kids clubs aren't as much fun--most kids hang out around ship during the day and go to clubs at night where I think the scene may have been too old for DS at the time.
When DS was done with the pools (there are 4 "real" pools--the hot tubs on Oasis are the size of the DCL pools!) he would go check out flow rider, or go down to Central Park and get a sandwich, or watch the movies/shows in Aquatheater, or go get pizza or soda on Promenade. Honestly, I love Disney and the Dream, but I would not do a 7 night cruise--there just isn't enough to do unless you spend all day in ports each stop. On the Oasis there will be TONS to do for all members and at 13, and together, I would feel comfortable letting them wander a bit on their own during the day.
Happy to answer any other specific questions--good luck!
