The 18-yr-old is welcome to go into Vibe during Open House times. But otherwise, as others have posted, it is off-limits to those over 17. As Maggie'sMom mentioned, the teens often just meet up at Vibe and do other activities together around the ship. Your daughter might be able to join her brother for some of those times. There is also the 1820 Society for passengers age 18-20; they don't have their own "space" but often meet in a lounge or nightclub. The activities they plan will vary based on who of that age is on the ship and their interests, but it might be lunch, port excursions, sports deck, movie, or simply hanging out.
Enjoy your cruise!