Have you given the Ruby and Sapphire to Celio?
Obviously, you've got the Ruby, otherwise you wouldn't have the Rainbow Pass. And I assume you've beaten Team Rocket in Icefall Cave, otherwise Lorelei would still be in there.....
You need to go to somewhere, I think one Six Island, to get the Sapphire. It's in a sort of valley, with a ruin involving Braille (you need Cut to get in) then a puzzle involving falling.A scientist will steal it. Then go to Five Island, and defeat all of Team Rocket in Five Isle Meadow.
Once you've done all of those things, Lorelei should return to the league, and you'll be wishing you'd levelled up more xD